A senior White House official confirmed on Tuesday that the Assad regime used sarin gas in a chemical attack against its own citizens this month.
Physiological samples from victims of the attack were "very consistent with nerve agent and sarin exposure," the official said. "Victimology also shows that those people don't have other wounds that would be consistent with another attack."
"We saw miosis. We saw frothing at the nose and mouth [and] twitching," the official explained. "All of those are consistent with nerve agent, they are not consistent with chlorine."
Secondary responders who helped rescue victims from affected areas and brought them to medical care "also had symptoms of sarin exposure."
The White House said Syria and Russia have a history of "trying to obfuscate the nature" of attacks like this and this is "an opportunity ... for the Russians to choose to stop the disinformation campaign and to make the commitment to accept what happened and work forward to eliminate weapons of mass destruction from Syria together.
"In terms of the Russian narrative … across the board, starting in 2013 and then since, we've seen both the Russians and the Syrians have a very clear campaign to try to obfuscate the nature of attacks, the attackers and what has happened in each particular incident."
"The cover-up is the disinformation that has happened since the day of the attack," the official added. "The absolute mass of data we have and all the different vehicles we've gotten... it is just too massive for really any intelligence organization to fabricate in that period of time," the official said.
The chemical attack on civilians in Idlib province on April 4 was the pre-text for President Trump's cruise missile strike on Syria's Shayrat airfield last week. Two U.S. Navy destroyers launched 59 Tomahawk cruise missiles into the airfield and the Pentagon believes about 20 Syrian planes were destroyed.
Russian President Vladimir Putin on Tuesday said he believes the U.S. plans to stage a chemical weapons attack in Syria and blame it on Assad.
"We have information that a similar provocation is being prepared ... in other parts of Syria including in the southern Damascus suburbs where they are planning to again plant some substance and accuse the Syrian authorities of using [chemical weapons]," Putin said Tuesday.
Putin said he plans to ask the United Nations to investigate the chemical weapons attack.
The White House official said it is "incredibly important that we speak with one voice at the United Nations ... that chemical weapons use of any kind by any actor is absolutely intolerable."
"We're working with all our partners and we've made it clear to the Russians as well that we support no case for the use of chemical weapons," the senior White House official said.