US official: Russia knew in advance that Syria would launch chemical weapons attack
Both Moscow and Damascus have denied that Assad dropped the chemical weapons, claiming instead that the gas was released accidentally when a Syrian air strike hit a "terrorist warehouse" containing "toxic substances."
Experts quickly cast doubt on Russia's explanation for Syria's worst chemical weapons 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 Syria's capital, Damascus. Assad still denies responsibility for that attack.
The US determined shortly after the attack occurred that Syrian warplanes had dropped the chemical weapons, 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."
British Foreign Secretary Boris Johnson said "all the evidence I have seen suggests this was the Assad regime ... using illegal weapons on their own people."
The US retaliated against 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 chemical weapons.
US Secretary of State Rex Tillerson said that Russia, which helped broker the deal to destroy Assad's chemical weapons stockpile in 2013, "has been complicit" or "simply incompetent in its ability to deliver on its end of the agreement."
This story is developing.