Syrian President Bashar Assad, left, and Russian President Vladimir Putin. AFP/Reuters/Amanda Macias/Business Insider 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.

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.

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.