US airports, cities on high alert after deadly bombings in Belgium

U.S. Airports Beef Up Security in Aftermath of Brussels Attacks

NEW YORK (Reuters) -- Airports in major U.S. cities were on high alert on Tuesday, with police out in force after at least 30 people were killed in attacks on a Brussels airport and subway, though officials said there was no evidence of specific threats to the United States.

READ MORE: Delta, United reroute flights from Brussels after blasts

U.S. President Barack Obama and Democratic presidential election contender Hillary Clinton vowed to do more to take on militants, while Republican presidential front-runner Donald Trump called for tighter border security and suggested U.S. intelligence services could use torture to head off future attacks.

More reports from social media about domestic security:

28 PHOTOS
Brussels Attacks: Domestic security response
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US airports, cities on high alert after deadly bombings in Belgium
NEW YORK, NY - MARCH 22: Heavily armed police patrol the streets in lower Manhattan following a heightened terror alert after attacks in the Belgian capital of Brussels on March 22, 2016 in New York City. Dozens are thought to have been killed after numerous bombs were set off at the Brussels airport and Metro. (Photo by Spencer Platt/Getty Images)
NEW YORK, NY - MARCH 22: Heavily armed police patrol the streets in lower Manhattan following a heightened terror alert after attacks in the Belgian capital of Brussels on March 22, 2016 in New York City. Dozens are thought to have been killed after numerous bombs were set off at the Brussels airport and Metro. (Photo by Spencer Platt/Getty Images)
NEW YORK, NY - MARCH 22: Heavily armed police patrol the streets in lower Manhattan following a heightened terror alert after attacks in the Belgian capital of Brussels on March 22, 2016 in New York City. Dozens are thought to be killed after numerous bombs were set off at the Brussels airport and Metro. (Photo by Spencer Platt/Getty Images)
New York Police Department (NYPD) officers stand guard in Times Square on March 22, 2016, in New York. New York and Washington stepped up security in the wake of the attacks in Brussels on March 22, deploying counter-terrorism reinforcements and the National Guard to airports and stations, officials said. The New York Police Department (NYPD) said there was no indication that the attacks in Belgium were connected to New York, but ordered the steps as America's biggest city of 8.4 million began the morning commute. Around 35 people were killed and more than 200 wounded on Tuesday in bombings at the Brussels airport and a metro station in the city that is home to the European Union and NATO. / AFP / Jewel SAMAD (Photo credit should read JEWEL SAMAD/AFP/Getty Images)
New York Police Department (NYPD) officers stand guard in Times Square on March 22, 2016, in New York. New York and Washington stepped up security in the wake of the attacks in Brussels on March 22, deploying counter-terrorism reinforcements and the National Guard to airports and stations, officials said. The New York Police Department (NYPD) said there was no indication that the attacks in Belgium were connected to New York, but ordered the steps as America's biggest city of 8.4 million began the morning commute. Around 35 people were killed and more than 200 wounded on Tuesday in bombings at the Brussels airport and a metro station in the city that is home to the European Union and NATO. / AFP / Jewel SAMAD (Photo credit should read JEWEL SAMAD/AFP/Getty Images)
A New York Police Department (NYPD) officer patrols in Times Square on March 22, 2016, in New York. New York and Washington stepped up security in the wake of the attacks in Brussels on March 22, deploying counter-terrorism reinforcements and the National Guard to airports and stations, officials said. The New York Police Department (NYPD) said there was no indication that the attacks in Belgium were connected to New York, but ordered the steps as America's biggest city of 8.4 million began the morning commute. Around 35 people were killed and more than 200 wounded on Tuesday in bombings at the Brussels airport and a metro station in the city that is home to the European Union and NATO. / AFP / Jewel SAMAD (Photo credit should read JEWEL SAMAD/AFP/Getty Images)
A New York Police Department (NYPD) officer patrols in Times Square on March 22, 2016, in New York. New York and Washington stepped up security in the wake of the attacks in Brussels on March 22, deploying counter-terrorism reinforcements and the National Guard to airports and stations, officials said. The New York Police Department (NYPD) said there was no indication that the attacks in Belgium were connected to New York, but ordered the steps as America's biggest city of 8.4 million began the morning commute. Around 35 people were killed and more than 200 wounded on Tuesday in bombings at the Brussels airport and a metro station in the city that is home to the European Union and NATO. / AFP / Jewel SAMAD (Photo credit should read JEWEL SAMAD/AFP/Getty Images)
New York Police Department (NYPD) officers patrol in Times Square on March 22, 2016, in New York. New York and Washington stepped up security in the wake of the attacks in Brussels on March 22, deploying counter-terrorism reinforcements and the National Guard to airports and stations, officials said. The New York Police Department (NYPD) said there was no indication that the attacks in Belgium were connected to New York, but ordered the steps as America's biggest city of 8.4 million began the morning commute. Around 35 people were killed and more than 200 wounded on Tuesday in bombings at the Brussels airport and a metro station in the city that is home to the European Union and NATO. / AFP / Jewel SAMAD (Photo credit should read JEWEL SAMAD/AFP/Getty Images)
New York Police Department (NYPD) officers stand guard in Times Square on March 22, 2016, in New York. New York and Washington stepped up security in the wake of the attacks in Brussels on March 22, deploying counter-terrorism reinforcements and the National Guard to airports and stations, officials said. The New York Police Department (NYPD) said there was no indication that the attacks in Belgium were connected to New York, but ordered the steps as America's biggest city of 8.4 million began the morning commute. Around 35 people were killed and more than 200 wounded on Tuesday in bombings at the Brussels airport and a metro station in the city that is home to the European Union and NATO. / AFP / Jewel SAMAD (Photo credit should read JEWEL SAMAD/AFP/Getty Images)
NEW YORK, NY - MARCH 22: Heavily armed police patrol the streets in lower Manhattan following a heightened terror alert after attacks in the Belgian capital of Brussels on March 22, 2016 in New York City. Dozens are thought to be killed after numerous bombs were set off at the Brussels airport and Metro. (Photo by Spencer Platt/Getty Images)
Out in full force. Protecting NYC with the #Belgiumattack in mind. Rest assured we are prepared. https://t.co/SRLwsYxihn
#LAXPD has increased patrol in all terminals. Report Suspicious Activity To The Police. https://t.co/aKfOsyY3q3
BREAKING: NYPD to ramp up security at mass transit points, bridges & tunnels, and other landmarks following today's attacks in Brussels.
Full statement on the #Belgium terror attacks. Stepping up NYPD patrols in transit & high profile areas in NYC. https://t.co/BCvrJIH5ZO
Explosions in #Brussels - Always stay alert. If you see something, say something. (888)NYC-SAFE https://t.co/or1qVjY0ZK
NYPD steps up security at @TimesSquareNYC due to Brussels attacks https://t.co/AcKgK8uaaP
DHS will not hesitate to adjust our security posture, as appropriate, to protect the American people. #Brussels
DHS is closely monitoring the unfolding events in #Brussels and we remain in contact with our counterparts in the region.
Heavy police presence and k9 In penn station
Seeing cops in riot gear w/ automatic rifles as I get off at Penn Station. Times like this I hate working close to a landmark. Be safe.
STAY STRONG #BRUSSELS. OUR HEARTS ARE WITH YOU-- https://t.co/yBJ1dGhgiD
Transit Police, including bomb dogs, conducting extra patrols on MAX and transit stations after #Brussels🇧🇪 attack. No known threat to #PDX.
LOS ANGELES, CA - MARCH 22: LAX airport police officers with automatic weapons patrol near the Tom Bradley International Terminal March 22, 2016 in Los Angeles, California. A terrorist attack in Brussels has put law enforcement on high alert in Los Angeles despite no specific threats. (Photo by Brian van der Brug/Los Angeles Times via Getty Images)
LOS ANGELES, CA - MARCH 22: Los Angeles County sheriff's transit police stand by the entrance to the Metro Red and Purple lines in Los Angeles Tuesday morning, March 22, 2016, as an added measure of security following terrorist attacks in Brussels. (Photo by Irfan Khan/Los Angeles Times via Getty Images)
LOS ANGELES, CA - MARCH 22: LAX airport police officer Cortinas keeps watch at Terminal Two March 22, 2016 in Los Angeles, California. A terrorist attack in Brussels has put law enforcement on high alert in Los Angeles despite no specific threats. (Photo by Brian van der Brug/Los Angeles Times via Getty Images)
LOS ANGELES, CA - MARCH 22: Los Angeles County sheriff's transit police patrol Union Station in downtown Los Angeles Tuesday morning, March 22, 2016, as an added measure of security following terrorist attacks in Brussels. (Photo by Irfan Khan/Los Angeles Times via Getty Images)
LOS ANGELES, CA - MARCH 22: Los Angeles County sheriff's transit police stand by the entrance to the Metro Red and Purple lines in Los Angeles Tuesday morning, March 22, 2016, as an added measure of security following terrorist attacks in Brussels. (Photo by Irfan Khan/Los Angeles Times via Getty Images)
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The Obama administration was expected to tighten security at U.S. airports following the Brussels airport attack, which occurred in a public hall outside of the security check area.

U.S. Representative Adam Schiff of California, the ranking Democrat on the House intelligence committee, said he was concerned that there was a risk of copy cat attacks and that airport security was still a concern despite changes in recent years.

"Overall though there's a lot more work we still need to do at our airports," Schiff told MSNBC.

RELATED: The devastating scenes in Brussels

35 PHOTOS
Brussels Attacks: Explosions at airport, Maelbeek metro station
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US airports, cities on high alert after deadly bombings in Belgium
People are evacuated from Brussels Airport, in Zaventem, on March 22, 2016. after at least 13 people have been killed by two explosions in the departure hall of Brussels Airport. / AFP / Belga / VIRGINIE LEFOUR / Belgium OUT (Photo credit should read VIRGINIE LEFOUR/AFP/Getty Images)
#BREAKING: Two loud explosions at #Zaventem airport in #Brussels https://t.co/JFw9RGLjnh
This video footage captures the aftermath of the explosions at Zaventem airport, #Brussels https://t.co/wcsNygh91Y
Brussels aftermath video shows survivors huddled together, telling one another: "It's okay". https://t.co/3Y7lEXrrjP https://t.co/KdqFXR9Wft
BRUSSELS, BELGIUM - MARCH 22: Passengers are evacuated from Zaventem Bruxelles International Airport after a terrorist attack on March 22, 2016 in Brussels, Belgium. At least 13 people are though to have been killed after Brussels airport was hit by two explosions whilst a Metro station was also targeted. The attacks come just days after a key suspect in the Paris attacks, Salah Abdeslam, was captured in Brussels. (Photo by Sylvain Lefevre/Getty Images)
A private security guard helps a wounded women outside the Maalbeek metro station in Brussels on March 21, 2016 after a blast at this station located near the EU institutions. Belgian firefighters said at least 26 people had died after 'enormous' blasts rocked Brussels airport and a city metro station today, as Belgium raised its terror threat to the maximum level. / AFP / Michael VILLA (Photo credit should read MICHAEL VILLA/AFP/Getty Images)
A picture taken on March 22, 2016 shows a Belgian police vehicle driving past passengers who are evacuating the Brussels Airport of Zaventem. At least 13 people were killed and 35 injured as twin blasts rocked the main terminal of Brussels airport on March 22, 2016, Belgian media quoted the federal prosecutor as saying. Officials were not immediately available to confirm the figure when contacted by AFP. The federal police earlier confirmed one death. / AFP / Belga / Jonas Roosens / Belgium OUT (Photo credit should read JONAS ROOSENS/AFP/Getty Images)
A picture taken on March 22, 2016 shows a Belgian police vehicle driving past passengers who are evacuating the Brussels Airport of Zaventem. At least 13 people were killed and 35 injured as twin blasts rocked the main terminal of Brussels airport on March 22, 2016, Belgian media quoted the federal prosecutor as saying. Officials were not immediately available to confirm the figure when contacted by AFP. The federal police earlier confirmed one death. / AFP / Belga / Jonas Roosens / Belgium OUT (Photo credit should read JONAS ROOSENS/AFP/Getty Images)
A picture taken on March 22, 2016 shows smoke rising from the Maalbeek underground, in Brussels, following a blast at the station close to the capital's European quarter. The Brussels metro service was being shut down on March 22, its operator said. The measure came after a rush-hour explosion at Maalbeek station with TV images showing black smoke pouring from the station entrance. / AFP / Belga / Seppe KNAPEN / Belgium OUT (Photo credit should read SEPPE KNAPEN/AFP/Getty Images)
A security perimeter has been set, on March 22, 2016 near Maalbeek metro station in Brussels, after a blast at this station near the EU institutions caused deaths and injuries. AFP PHOTO / EMMANUEL DUNAND / AFP / EMMANUEL DUNAND (Photo credit should read EMMANUEL DUNAND/AFP/Getty Images)
BRUSSELS, BELGIUM - MARCH 22: Broken glass and blood is seen beside signage outside an entrance to Maelbeek metro station following todays attack on March 22, 2016 in Brussels, Belgium. At least 34 people are thought to have been killed after Brussels airport and a Metro station were targeted by explosions. The attacks come just days after a key suspect in the Paris attacks, Salah Abdeslam, was captured in Brussels. (Photo by Carl Court/Getty Images)
BRUSSELS, BELGIUM - MARCH 22: Broken glass is seen outside an entrance to Maelbeek metro station following todays attack on March 22, 2016 in Brussels, Belgium. At least 34 people are thought to have been killed after Brussels airport and a Metro station were targeted by explosions. The attacks come just days after a key suspect in the Paris attacks, Salah Abdeslam, was captured in Brussels. (Photo by Carl Court/Getty Images)
Policemen stand guard at the entrance of a security perimeter set near Maalbeek metro station, on March 22, 2016 in Brussels, after a blast at this station near the EU institutions caused deaths and injuries. AFP PHOTO / EMMANUEL DUNAND / AFP / EMMANUEL DUNAND (Photo credit should read EMMANUEL DUNAND/AFP/Getty Images)
Rescue teams evacuate wounded people outside the Maalbeek metro station in Brussels on March 22, 2016 after a blast at this station located near the EU institutions. Belgian firefighters said at least 26 people had died after 'enormous' blasts rocked Brussels airport and a city metro station today, as Belgium raised its terror threat to the maximum level. / AFP / - (Photo credit should read -/AFP/Getty Images)
Passengers evacuated after blast at #Maalbeek metro station in #Brussels via @EurActiv https://t.co/O3W8C2rpPm https://t.co/XmIoeKfE9F
People are evacuated from Brussels airport in Zaventem on March 22, 2016 following twin blasts. A string of explosions rocked Brussels airport and a city metro station, killing at least 21 people in apparently coordinated attacks, officials said. == BELGIUM OUT == / AFP / Belga / DIRK WAEM / Belgium OUT (Photo credit should read DIRK WAEM/AFP/Getty Images)
A man with blood stains on his sweater leaves Brussels airport in Zaventem on March 22, 2016 following twin blasts. A string of explosions rocked Brussels airport and a city metro station, killing at least 21 people in apparently coordinated attacks, officials said. == BELGIUM OUT == / AFP / Belga / DIRK WAEM / Belgium OUT (Photo credit should read DIRK WAEM/AFP/Getty Images)
Amateur footage shows scene at #Brussels airport after explosions (video: Bart van Meele) https://t.co/v29k9dvMiM
Passengers are evacuated from Brussels airport, on March 22, 2016 in Zaventem, after at least 13 people were killed and 35 injured as twin blasts rocked the main terminal of Brussels airport.AFP PHOTO / JOHN THYS / AFP / JOHN THYS (Photo credit should read JOHN THYS/AFP/Getty Images)
A victim receives first aid by rescuers, on March 22, 2016 near Maalbeek metro station in Brussels, after a blast at this station near the EU institutions caused deaths and injuries. AFP PHOTO / EMMANUEL DUNAND / AFP / EMMANUEL DUNAND (Photo credit should read EMMANUEL DUNAND/AFP/Getty Images)
A victim receives first aid by rescuers, on March 22, 2016 near Maalbeek metro station in Brussels, after a blast at this station near the EU institutions caused deaths and injuries. AFP PHOTO / EMMANUEL DUNAND / AFP / EMMANUEL DUNAND (Photo credit should read EMMANUEL DUNAND/AFP/Getty Images)
BRUSSELS, BELGIUM - MARCH 22 : Police officers stand guard around the Zaventem Airport after two explosions went off in Brussels, Belgium on March 22, 2016. (Photo by Dursun Aydemir/Anadolu Agency/Getty Images)
BRUSSELS, BELGIUM - MARCH 22 : Police officers stand guard around the Zaventem Airport after two explosions went off in Brussels, Belgium on March 22, 2016. (Photo by Dursun Aydemir/Anadolu Agency/Getty Images)
Passengers are gathered, on March 22, 2016 near Brussels airport in Zaventem, following its evacuation after at least 13 people were killed and 35 injured as twin blasts rocked the main terminal of Brussels airport.AFP PHOTO / JOHN THYS / AFP / JOHN THYS (Photo credit should read JOHN THYS/AFP/Getty Images)
BRUSSELS, BELGIUM - MARCH 22 : Police officers stand guard around the Zaventem Airport after two explosions went off in Brussels, Belgium on March 22, 2016. (Photo by Dursun Aydemir/Anadolu Agency/Getty Images)
Firefighters arrive at a security perimeter set in the Rue de la Loi near the Maalbeek subway station, in Brussels, on March 22, 2016, after an explosion killed around 10 people, according to spokesman of Brussels' fire brigade A string of explosions rocked Brussels airport and a city metro station on Tuesday, killing at least 13 people, according to media reports, as Belgium raised its terror threat to the maximum level. / AFP / Belga / LAURIE DIEFFEMBACQ / Belgium OUT (Photo credit should read LAURIE DIEFFEMBACQ/AFP/Getty Images)
Passengers who were evacuated from the airport wait in Zaventem, on March 22, 2016, after a string of explosions rocked Brussels airport of Zaventem and a city metro station, killing at least 13 people, according to media reports, as Belgium raised its terror threat to the maximum level. / AFP / Belga / SEPPE KNAPEN / Belgium OUT (Photo credit should read SEPPE KNAPEN/AFP/Getty Images)
BRUSSELS, BELGIUM - MARCH 22 : Police officers stand guard around the Zaventem Airport after two explosions went off in Brussels, Belgium on March 22, 2016. (Photo by Dursun Aydemir/Anadolu Agency/Getty Images)
People are evacuated from Brussels Airport, in Zaventem, on March 22, 2016. after at least 13 people have been killed by two explosions in the departure hall of Brussels Airport. / AFP / Belga / VIRGINIE LEFOUR / Belgium OUT (Photo credit should read VIRGINIE LEFOUR/AFP/Getty Images)
Passengers gather, on March 22, 2016 near Brussels airport in Zaventem , following its evacuation after at least 13 people were killed and 35 injured as twin blasts rocked the main terminal of Brussels airport.AFP PHOTO / JOHN THYS / AFP / JOHN THYS (Photo credit should read JOHN THYS/AFP/Getty Images)
Passengers leave with their luggages, on March 22, 2016 near Brussels airport in Zaventem , following its evacuation after at least 13 people were killed and 35 injured as twin blasts rocked the main terminal of Brussels airport.AFP PHOTO / JOHN THYS / AFP / JOHN THYS (Photo credit should read JOHN THYS/AFP/Getty Images)
BRUSSELS, BELGIUM - MARCH 22 : Police officers stand guard around the Zaventem Airport after two explosions went off in Brussels, Belgium on March 22, 2016. (Photo by Dursun Aydemir/Anadolu Agency/Getty Images)
A photo taken on March 22, 2016 shows Brussels Airport, in Zaventem, after two explosions rocked the main hall of the airport. At least 13 people have been killed after two explosions occured in the departure hall of Brussels Airport. Government sources speak of a terrorist attack. The terrorist threat level has been heightened to four across the country. / AFP / BELGA / DIRK WAEM / Belgium OUT (Photo credit should read DIRK WAEM/AFP/Getty Images)
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Islamic State, a militant group that has gained control of large areas of Iraq and Syria and has sympathizers and supporters around the world, claimed responsibility. Amaq, a news agency affiliated to the group, said suicide bombers strapped into explosive belts had staged the attacks on the airport and a rush-hour metro train in the Belgian capital.

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