WASHINGTON — A senior US official said Monday that the United States has concluded that Russia knew in advance of Syria’s chemical weapons attack last week that killed over 80 people, including dozens of children.

The official said a drone operated by Russians was flying over a hospital as victims of the attack were rushing to get treatment. Hours after the drone left, a Russian-made fighter jet bombed the hospital in what American officials believe was an attempt to cover up the usage of chemical weapons.

The official said the presence of the surveillance drone over the hospital 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 official, who wasn’t authorized to speak publicly on intelligence matters and demanded anonymity, didn’t give precise timing for when the drone was above the northern Syrian town of Khan Sheikhoun. He also didn’t provide all the details for the military and intelligence information that form the basis of what he said the Pentagon has now concluded.

The allegation is grave, even by the standards of the currently dismal US-Russian relations.

Although Russia has steadfastly supported Syrian President Bashar Assad’s government, and they’ve coordinated military attacks together, Washington has never previously accused Moscow of complicity in any attack that involved the gassing of innocent civilians, including children. The former Cold War foes even worked together in 2013 to remove and destroy more than 1,300 tons of Syrian chemical weapons and agents.

Until Monday, US officials had said they weren’t sure whether Russia or Syria operated the drone. The official said the US is now convinced Russia controlled the drone. The official said it still isn’t clear who was flying the jet that bombed the hospital, because the Syrians also fly Russian-made aircraft.

US officials previously have said Russians routinely work with Syrians at the Shayrat air base where the attack is supposed to have originated. US officials say the chemical weapons were stored there.

Those elements, the senior official said, add to the conclusion that Russia was complicit in the attack to which the US responded by firing 59 Tomahawk missiles were fired on the government-controlled base, in the United States’ first direct military action against Assad’s forces.

Earlier Monday, US Senator John McCain accused Russia of having cooperated with Syrian government forces in the chemical weapons attack last week.

The Republican senator said at a press conference in Belgrade that he believes “the Russians knew about chemical weapons because they were operating exactly from the same base.”

Sen. John McCain (R-AZ) answers questions at the US Capitol about the recent US attack in Syria April 7, 2017 in Washington, DC. (Win McNamee/Getty Images/AFP)

Sen. John McCain (R-AZ) answers questions at the US Capitol about the recent US attack in Syria April 7, 2017 in Washington, DC. (Win McNamee/Getty Images/AFP)

“I hope that this behavior by Syria, in what clearly is cooperation with Russia and Syria together, will never happen again,” he said.

McCain said the US should take out Syria’s air force as part of stopping Assad from repeating such attacks in the future.

“I would prevent Bashar Assad from flying from his airfields if he doesn’t renounce the use of these weapons,” the former American airman said. “The United States should first tell Russia that this kind of a war crime is unacceptable in the world today.”

He said the upcoming visit by US Secretary of State Rex Tillerson to Moscow should “lead to an agreement on the part of the Russians that they will not allow Syria to ever again use chemical weapons.”

“Remember, Russians made that commitment after they (Syrian government forces) crossed the so-called red line back in 2014. I hope that this will be the first item on the agenda,” McCain said.

He also said he would “make sure that we arm and train some of those who fight against Bashar Assad.”

McCain is on a tour of the western Balkans, the war-weary European region where Russia has been vying for increased military, political and economic influence.