US official: Russia knew in advance that Syria would launch chemical-weapons attack

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The US has concluded that Russia knew in advance about the chemical attack in Syria last week that killed dozens of civilians and is believed to have been carried out by Syrian President Bashar Assad's forces, a senior US official told The Associated Press on Monday.

Both Moscow and Damascus have denied that Assad's forces dropped the chemicals, claiming that the gas was released accidentally when a Syrian airstrike hit a "terrorist warehouse" containing "toxic substances."

Experts quickly cast doubt on Russia's explanation for Syria's worst chemical attack since 2013, when Assad, an ally of Russian President Vladimir Putin, is believed to have used sarin gas to kill as many as 1,400 people in the outskirts of Damascus, Syria's capital. Assad still denies responsibility for that attack.

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Russian forces in Syria
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Russian forces in Syria
Russian soldiers walk in the Old City of Aleppo, Syria January 31, 2017. REUTERS/Ali Hashisho
A Russian soldier walks to a military vehicle in goverment controlled Hanono housing district in Aleppo, Syria December 4, 2016. REUTERS/Omar Sanadiki
Russian soldiers carry their weapons in the Old City of Aleppo, Syria January 31, 2017. REUTERS/Ali Hashisho
Russian soldiers, on armoured vehicles, patrol a street in Aleppo, Syria February 2, 2017. REUTERS/Ali Hashisho
Men inspect the wreckage of a Russian helicopter that had been shot down in the north of Syria's rebel-held Idlib province, Syria August 1, 2016. REUTERS/Ammar Abdullah TPX IMAGES OF THE DAY
Rebel fighters stand in line past Russian soldiers (back) as they wait to evacuate the besieged Waer district in the central Syrian city of Homs, after an agreement was reached between rebels and Syria's army, March 18, 2017. REUTERS/Omar Sanadiki
Russian soldiers, on armored vehicles, patrol a street in Aleppo, Syria February 2, 2017. Picture taken February 2, 2017. REUTERS/Ali Hashisho
A man looks towards a Russian helicopter as it flies over ruins in the historic city of Palmyra, Syria March 4, 2017. REUTERS/Omar Sanadiki
A Russian soldier stands near a bus carrying people who came back to inspect their homes in government controlled Hanono housing district in Aleppo, Syria December 4, 2016. REUTERS/Omar Sanadiki
Russian soldiers stand near food aid being distributed to Syrians evacuated from eastern Aleppo, in government controlled Jibreen area in Aleppo, Syria November 30, 2016. The text on the bag, showing Syrian and Russian national flags, reads in Arabic: "Russia is with you". REUTERS/Omar Sanadiki
Russian soldiers and civilians walk along a street in Aleppo, Syria January 30, 2017. Picture taken January 30, 2017. REUTERS/Ali Hashisho
A Russian soldier drives a military vehicle in Aleppo, Syria December 4, 2016. REUTERS/Omar Sanadiki
Russian soldiers, on armoured vehicles, patrol a street in Aleppo, Syria February 2, 2017. REUTERS/Omar Sanadiki
Residents look at Russian vehicles in Aleppo, Syria December 4, 2016. REUTERS/Omar Sanadiki
Russian soldiers gather as rebel fighters and their families evacuate the besieged Waer district in the central Syrian city of Homs, after an agreement reached between rebels and Syria's army, Syria March 18, 2017. REUTERS/Omar Sanadiki
Russian soldiers, on armoured vehicles, patrol a street in Aleppo, Syria February 2, 2017. REUTERS/Omar Sanadiki
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The US determined shortly after the attack last week that Syrian warplanes had dropped the chemicals, which caused injuries and deaths that the World Health Organization said were "consistent with exposure to organophosphorus chemicals, a category of chemicals that includes nerve agents."

The Pentagon had been looking into whether Russia was complicit in the attack. A Russian drone was reportedly hovering above a hospital treating victims and then turned off just before the hospital was bombed. US officials believe the hospital was targeted in an attempt to hide evidence of the chemical attack.

Those officials have now concluded that the drone that was hovering above the hospital was operated by Russia, and the warplane that attacked the hospital was Russian-made. But they have not determined whether the plane was operated by a Russian or Syrian pilot.

"The official said the presence of the drone couldn't have been a coincidence, and that Russia must have known the chemical weapons attack was coming and that victims were seeking treatment," the AP reported.

The US retaliated against Syria for the gas attack on Thursday night, launching 59 Tomahawk cruise missiles at the airfield where Assad's warplanes are believed to have taken off, loaded with the chemicals.

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Syria gas attack
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ATTENTION EDITORS - VISUAL COVERAGE OF SCENES OF INJURY OR DEATH A man carries the body of a dead child, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun in rebel-held Idlib, Syria April 4, 2017. REUTERS/Ammar Abdullah
ATTENTION EDITORS - VISUAL COVERAGE OF SCENES OF INJURY OR DEATH People stand near a dead body, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun in rebel-held Idlib, Syria April 4, 2017. REUTERS/Ammar Abdullah
A man breathes through an oxygen mask as another one receives treatments, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun in rebel-held Idlib, Syria April 4, 2017. REUTERS/Ammar Abdullah
A man breathes through an oxygen mask, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun in rebel-held Idlib, Syria April 4, 2017. REUTERS/Ammar Abdullah
ATTENTION EDITORS - VISUAL COVERAGE OF SCENES OF INJURY OR DEATH Men stand near dead bodies, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun in rebel-held Idlib, Syria April 4, 2017. REUTERS/Ammar Abdullah
EDITORS NOTE: Graphic content / A Syrian child receives treatment at a hospital in Khan Sheikhun, a rebel-held town in the northwestern Syrian Idlib province, following an attack on April 4, 2017. A suspected chemical attack killed at least 58 civilians including several children in rebel-held northwestern Syria, a monitor said, with the opposition accusing the government and demanding a UN investigation. / AFP PHOTO / Omar haj kadour (Photo credit should read OMAR HAJ KADOUR/AFP/Getty Images)
A man holds an injured baby inside a Turkish ambulance as injured Syrian people enter into Turkey from the Cilvegozu border gate in Hatay province, near the Syrian border on April 4, 2017. Turkish President Recep Tayyip Erdogan in a phone call with Russian counterpart Vladimir Putin on April 4, 2017 condemned a suspected chemical attack in northwestern Syria as an 'inhuman' strike that could endanger peace talks based in the Kazakh capital. / AFP PHOTO / DOGAN NEWS AGENCY / - / Turkey OUT (Photo credit should read -/AFP/Getty Images)
IDLIB, SYRIA - APRIL 4: A wounded kid receives medical treatment at sahra hospital after Assad Regime forces's attack with chlorine gas to Khan Shaykhun town of Idlib, Syria on April 4, 2017. (Photo by Abdulghani Arian/Anadolu Agency/Getty Images)
IDLIB, SYRIA - APRIL 4: A woman gets treatment at a hospital after Assad Regime forces attacked with suspected chlorine gas to Khan Shaykhun town of Idlib, Syria on April 4, 2017. (Photo by Bahjat Najar/Anadolu Agency/Getty Images)
IDLIB, SYRIA - APRIL 4: A child gets treatment at a hospital after Assad Regime forces attacked with suspected chlorine gas to Khan Shaykhun town of Idlib, Syria on April 4, 2017. (Photo by Bahjat Najar/Anadolu Agency/Getty Images)
ATTENTION EDITORS - VISUAL COVERAGE OF SCENES OF INJURY OR DEATH A man carries the body of a dead child, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun in rebel-held Idlib, Syria April 4, 2017. REUTERS/Ammar Abdullah TPX IMAGES OF THE DAY
A crater is seen at the site of an airstrike, after what rescue workers described as a suspected gas attack in the town of Khan Sheikhoun in rebel-held Idlib, Syria April 4, 2017. REUTERS/Ammar Abdullah TPX IMAGES OF THE DAY
Turkish officials with chemical clothes carry a injured man on April 4, 2017 in Hatay province, near the Syrian border. Turkish President Recep Tayyip Erdogan in a phone call with Russian counterpart Vladimir Putin on April 4, 2017 condemned a suspected chemical attack in northwestern Syria as an 'inhuman' strike that could endanger peace talks based in the Kazakh capital. / AFP PHOTO / DOGAN NEWS AGENCY / - / Turkey OUT (Photo credit should read -/AFP/Getty Images)
EDITORS NOTE: Graphic content / Bodies lie in the parking area of a hospital in Khan Sheikhun, a rebel-held town in the northwestern Syrian Idlib province, following a suspected toxic gas attack on April 4, 2017. A suspected chemical attack killed at least 58 civilians including several children in rebel-held northwestern Syria, a monitor said, with the opposition accusing the government and demanding a UN investigation. / AFP PHOTO / Omar haj kadour (Photo credit should read OMAR HAJ KADOUR/AFP/Getty Images)
IDLIB, SYRIA - APRIL 4: A child gets treatment at a hospital after Assad Regime forces attacked with suspected chlorine gas to Khan Shaykhun town of Idlib, Syria on April 4, 2017. (Photo by Bahjat Najar/Anadolu Agency/Getty Images)
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US Secretary of State Rex Tillerson said on Thursday that Russia, which helped broker the deal in 2013 to destroy Assad's chemical-weapons stockpile, had been "complicit" or "simply incompetent in its ability to deliver on its end of that agreement."

It is unclear how the US will respond now that officials have concluded that Russia knew of the attack in advance. Defense Secretary Jim Mattis said on Monday that the Assad regime would be "ill-advised" to use chemical weapons in the future, foreshadowing a potentially greater US response than last week's strike.

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