WASHINGTON (Reuters) - Republican Senator Lindsey Graham on Friday called for immediate U.S. Senate hearings on President Donald Trump's decision to withdraw all American troops from Syria, which prompted the resignation of Defense Secretary Jim Mattis.
Graham, a member of the Senate Armed Services Committee, told reporters he wanted to hear directly from Mattis at any hearing. Mattis announced plans on Thursday to depart in a candid resignation letter to Trump that laid bare the growing divide between them.
A Senate hearing could also cover Trump administration officials saying on Thursday that there were plans to drawdown about 5,000 U.S. troops from Afghanistan.
Graham, who over the past year or so has been a staunch supporter of Trump, has broken with him on the Syria decision.
Heading to a meeting of Republican senators, Graham said, "In lunch I'm going to ask for hearings like right now about Syria." Trump said Islamic State had been defeated there so it was time to withdraw U.S. forces.
Graham made clear that he also was worried about a possible U.S. troop reduction in Afghanistan, where 14,000 troops are deployed in what is America's longest war at 17 years.
"I dare anybody to say that ISIS-K is defeated in Afghanistan," Graham said, referring to Islamic State of Iraq and the Levant – Khorasan Province, a branch of Islamic State, active in Afghanistan and Pakistan.
Photos of the week: 12/14 - 12/21
Photos of the week: 12/14 - 12/21
Part of Defense Secretary Jim Mattis' resignation letter to President Donald Trump is photographed in Washington, Thursday, Dec. 20, 2018. Mattis is stepping down from his post, Trump announced, after the retired Marine general clashed with the president over a troop drawdown in Syria and Trump's go-it-alone stance in world affairs. (AP Photo/Jon Elswick)
Fans leave tributes on Penny Marshall's star on the Hollywood Walk of Fame shortly after the news that the actress died aged 75 was made public, in Hollywood, California, on December 18, 2018. - Penny Marshall, the star of ABC's 'Laverne and Shirley' who became one of the most successful female directors in history at the helm of 'Big,' 'A League of Their Own' and a string of other hit movies, has died. She was 75. The filmmaker died peacefully in her Hollywood Hills on Monday, December 17, 2018 due to complications from diabetes, her publicist told AFP. (Photo by VALERIE MACON / AFP) (Photo credit should read VALERIE MACON/AFP/Getty Images)
Sen. Cory Booker, D-N.J., right, joins Sen. Chuck Grassley, R-Iowa, center, and Sen. Mike Lee, R-Utah, left, in an Instagram Live post before they participate in a news conference on Capitol Hill in Washington, Wednesday, Dec. 19, 2018, on prison reform legislation. A criminal justice bill passed in the Senate gives judges more discretion when sentencing some drug offenders and boosts prisoner rehabilitation efforts. (AP Photo/Susan Walsh)
PORT TALBOT, WALES - DECEMBER 20: People gather around fences that have been erected to protect the latest piece of artwork by the underground guerrilla artist Banksy on December 20, 2018 in Port Talbot, Wales. The British graffiti artist who keeps his identity a secret, confirmed yesterday that the artwork was his using his verified Instagram account to announce 'Season's greetings' with a video of the artwork in Port Talbot. (Photo by Matt Cardy/Getty Images)
Former U.S. national security adviser Michael Flynn passes by members of the media as he departs after his sentencing was delayed at U.S. District Court in Washington, U.S., December 18, 2018. REUTERS/Joshua Roberts TPX IMAGES OF THE DAY
Passengers wait to check in at Gatwick Airport in England, Friday, Dec. 21, 2018. Flights resumed at London's Gatwick Airport on Friday morning after drones sparked the shutdown of the airfield for more than 24 hours, leaving tens of thousands of passengers stranded or delayed during the busy holiday season. (AP Photo/Kirsty Wigglesworth)
NEW YORK, NEW YORK - DECEMBER 17: Traders work on the floor of the New York Stock Exchange (NYSE) on December 17, 2018 in New York City. Stocks fell again in morning trading as investors remain jittery over trade with China, interest rates and the threat of a global slowdown. (Photo by Spencer Platt/Getty Images)
A surfer exits a wave while surfing at Black's Beach Tuesday, Dec. 18, 2018, in San Diego. Large surf continued to pound the California coast Tuesday, as officials warned of dangerous currents along the beaches and hazardous conditions at sea. (AP Photo/Gregory Bull)
President Donald Trump walks with an umbrella from Section 60 of Arlington National Cemetery in Arlington, Va., Saturday, Dec. 15, 2018, after visiting during Wreaths Across America Day. Wreaths Across America was started in 1992 at Arlington National Cemetery by Maine businessman Morrill Worcester and has expanded to hundreds of veterans' cemeteries and other locations in all 50 states and beyond. (AP Photo/Carolyn Kaster)
Houses on fire are seen at Educandos neighbourhood, a branch of the Rio Negro, a tributary to the Amazon river, in the city of Manaus, Brazil December 17, 2018. Picture taken December 17, 2018. REUTERS/Bruno Kelly TPX IMAGES OF THE DAY
SYDNEY, AUSTRALIA - DECEMBER 21: A Gentoo penguin gets into the festive spirit of Christmas by interacting with Christmas baubles at Sea Life Sydney Aquarium on December 21, 2018 in Sydney, Australia. Part of their enrichment, keepers placed a dozen baubles onto the ice for the creatures to play with during the day. (Photo by James D. Morgan/Getty Images)
Former first lady Michelle Obama, left, is interviewed by Sarah Jessica Parker during an appearance for her book, "Becoming: An Intimate Conversation with Michelle Obama" at Barclays Center on Wednesday, Dec. 19, 2018, in New York. (AP Photo/Frank Franklin II)
OPA LOCKA, FLORIDA - DECEMBER 17: U.S. Postal service mail handler Nikeisha Mitchell sorts packages at the U.S. Postal service's Royal Palm Processing and Distribution Center on December 17, 2018 in Opa Locka, Florida. The U.S. Postal Service's busiest week of the year began Monday as the Christmas rush is on with the postal service expecting to deliver three billion pieces of mail this week, including greeting cards, before Christmas. (Photo by Joe Raedle/Getty Images)
Honduran asylum seekers are taken into custody by U.S. Border Patrol agents after the group crossed the U.S. border wall into San Diego, California, seen from Tijuana, Mexico, Saturday, Dec. 15, 2018. (AP Photo/Moises Castillo)
The Capitol is seen under early morning skies in Washington, Thursday, Dec. 20, 2018. The Senate approved legislation to temporarily fund the government late last night, a key step toward averting a federal shutdown after President Donald Trump backed off his demand for money for a border wall with Mexico. The House is expected to vote before Friday's deadline, when funding for a portion of the government expires. Without resolution, more than 800,000 federal workers would face furloughs or be forced to work without pay, disrupting government operations days before Christmas. (AP Photo/J. Scott Applewhite)
LHASA, CHINA - DECEMBER 18: Special police of Tibet Armed Police Corps take part in an anti-terrorism drill in snow on December 18, 2018 in Lhasa, Tibet Autonomous Region of China. (Photo by Yu Wenbin/VCG via Getty Images)
Elon Musk, co-founder and chief executive officer of Tesla Inc., speaks during an unveiling event for the Boring Company Hawthorne test tunnel in Hawthorne, south of Los Angeles, California on December 18, 2018. - Musk explained that the snail moves 14 times faster than a tunnel-digging machine.
On Tuesday night December 18, 2018, Boring Co. officially opened the Hawthorne tunnel, a preview of Elon Musk's larger vision to ease L.A. traffic. (Photo by Robyn Beck / POOL / AFP) (Photo credit should read ROBYN BECK/AFP/Getty Images)
A Santa Claus sculpture is fixed on the roof rack of a Volkswagen Beetle vintage car decorated with a light chain on December 19, 2018 in front of a house in Ertingen, southern Germany. (Photo by Thomas Warnack / dpa / AFP) / Germany OUT (Photo credit should read THOMAS WARNACK/AFP/Getty Images)
LONDON, UNITED KINGDOM - DECEMBER 19: (EMBARGOED FOR PUBLICATION IN UK NEWSPAPERS UNTIL 24 HOURS AFTER CREATE DATE AND TIME) Prince William, Duke of Cambridge, Catherine, Duchess of Cambridge and Prince George of Cambridge (accompanied by his nanny María Borrallo) attend a Christmas lunch for members of the Royal Family hosted by Queen Elizabeth II at Buckingham Palace on December 19, 2018 in London, England. (Photo by Max Mumby/Indigo/Getty Images)
Contestants congratulate new Miss Universe Catriona Gray, center, from Philippines during the final round of 67th Miss Universe competition in Bangkok, Thailand, Monday, Dec. 17, 2018.(AP Photo/Gemunu Amarasinghe)
A picture taken on December 17, 2018 in Herrlisheim shows jewish headstones tagged with swastika symbol at a Jewish cemetery, eastern France. - 37 headstones and a memorial for Shoah victims have been desacrated overnight December 10, 2018. (Photo by SEBASTIEN BOZON / AFP) (Photo credit should read SEBASTIEN BOZON/AFP/Getty Images)
A young silverback gorilla searches for food in an advent calendar as animals are given meals styled along a Christmas theme in London, Britain, December 20, 2018. REUTERS/Toby Melville TPX IMAGES OF THE DAY
Residents react during a gathering being held in a central square of the eastern French city of Strasbourg, Sunday Dec.16, 2018 to pay homage to the victims of a gunman who killed four people and wounded a dozen more. The gathering was held in Kleber Square by a Christmas market and near where the gunman opened fire last Tuesday evening. (AP Photo/Jean-Francois Badias)
Retiring U.S. Speaker of the House Paul Ryan arrives to deliver his farewell address in the Great Hall of the Library of Congress in Washington, U.S., December 19, 2018. REUTERS/Jonathan Ernst TPX IMAGES OF THE DAY
TWICKENHAM, ENGLAND - DECEMBER 18: Meghan, Duchess of Sussex speaks to head chef Stuart Jones as she visits the Royal Variety Charity's residential nursing and care home at Brinsworth House on December 18, 2018 in Twickenham, England. (Photo by Geoff Pugh- WPA Pool/Getty Images)
Harvey Weinstein removes his jacket as he passes through a security checkpoint upon arriving at New York Supreme Court, Thursday, Dec. 20, 2018, in New York. Judge James Burke will decide on the future of Weinstein's sexual assault case, which has been clouded by allegations that police acted improperly in the investigation that led to his arrest. (AP Photo/Julio Cortez)
Firefighters burn flammable material and a mock coffin, during a protest demanding better work conditions, in front of the Catalunya Parliament in Barcelona, Spain, Thursday, Dec. 20, 2018. (AP Photo/Emilio Morenatti)
A crew member of a cargo ship that ran aground carrying a suitcase is rescued with the help of a winch system extended from the shore, off the Black Sea coast of Sile, Turkey, Wednesday, Dec. 19, 2018. Turkish coastal safety teams have rescued 16 crew members from the in a dramatic operation that lasted several hours.The Comoros-flagged ship, Natalia, ran aground in rough seas early on Wednesday en route to Istanbul from Russia.(Akin Celiktas/DHA via AP)
Migrants, part of a caravan of thousands from Central America trying to reach the United States, rest at a temporary shelter near the U.S. border in Tijuana, Mexico December 19, 2018. REUTERS/Mohammed Salem TPX IMAGES OF THE DAY
KING'S LYNN, ENGLAND - DECEMBER 20: Queen Elizabeth II arrives by train at King's Lynn Station to begin the Christmas Holidays at Sandringham on December 20, 2018 in King's Lynn, England. (Photo by Mark Cuthbert/UK Press via Getty Images)
People skate on the ice rink opened on Manezhnaya Square decorated for Christmas and New Year celebrations near Red Square with the Kremlin Wall in the background in Moscow, Russia, Friday, Dec. 21, 2018. (AP Photo/Alexander Zemlianichenko)
A resident looks out at the scorched remains of the Educandos neighborhood in Manaus, Brazil, Tuesday, Dec. 18, 2018. Officials say the fire engulfed the neighborhood in the northern Brazilian city of Manaus and destroyed at least 600 wooden houses. (AP Photo/Edmar Barros)
Elvira Choc, 59, Jakelin Amei Rosmery Caal's grandmother, rests her head on her hand in front of her house in Raxruha, Guatemala, on Saturday, Dec. 15, 2018. The 7-year old girl died in a Texas hospital, two days after being taken into custody by border patrol agents in a remote stretch of New Mexico desert. (AP Photo/Oliver de Ros)
Immigrants participate in a naturalization ceremony to become U.S. citizens in Los Angeles, California, December 19, 2018. REUTERS/Lucy Nicholson TPX IMAGES OF THE DAY
The Polar Express makes it's last stop on the Roanoke City Farmer's Market for parade goers to admire as part of the Downtown Roanoke Christmas Parade on Friday, Dec. 14, 2018 in Roanoke, Va. The float was sponsored by the Western Virginia Water Authority and won first place. (Don Petersen/The Roanoke Times via AP)
WASHINGTON, DC - DECEMBER 20: Sen. Lindsey Graham (R-SC) speaks during a press conference at the U.S. Capitol on December 20, 2018 in Washington, DC. Graham and Sen. Robert Menendez (L) (R-NJ) and Sen. Jack Reed (D-RI) spoke out against U.S. President Donald Trump's decision to remove U.S. military forces from Syria. (Photo by Win McNamee/Getty Images)
HALF MOON BAY, CA - DECEMBER 17: A surfer rides a wave at Mavericks on December 17, 2018 in Half Moon Bay, California. A giant swell brought waves of up to 50 feet high to Northern California. (Photo by Ezra Shaw/Getty Images)
Italy's Dorothea Wierer skis past a holiday tree as she competes in the women's Biathlon World Cup 7.5 km sprint event in Nove Mesto na Morave, Czech Republic, Friday, Dec. 21, 2018. (AP Photo/Petr David Josek)
Police officers inspect the scene where a car plowed into a bus stop in Recklinghausen, Germany December 20, 2018. REUTERS/Leon Kuegeler TPX IMAGES OF THE DAY
HARBIN, CHINA - DECEMBER 19: Workers carve the main sculpture ahead of the Harbin Sun Island International Snow Sculpture Art Exposition on December 19, 2018 in Harbin, Heilongjiang Province of China. The snow sculpture is 33 meters high and over 100 meters long. (Photo by VCG/VCG via Getty Images)
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The group "is a bigger threat this year than they were last year. It is clear to me that ISIS-K is plotting to hit America," Graham said.
Graham said cutting U.S. troop strength in Afghanistan would leave "too few to accomplish the mission of holding Afghanistan together and protecting America from another attack and it's too many to be hostages and sitting ducks" there.
The United States went to war in Afghanistan in 2001 in the wake of the Sept. 11, 2001 attacks on the World Trade Center and the Pentagon, seeking to oust the Taliban militants harboring Saudi-raised militant Osama bin Laden, who led plans to carry out the attacks.
(Reporting by Richard Cowan; Writing by Susan Heavey; Editing by James Dalgleish and Grant McCool)