"It is our understanding the Russians may have visited the attack site", US Ambassador Kenneth Ward said at a meeting of the OPCW in The Hague on Monday.
In a joint briefing with U.S. officials, Mr Martin said they had seen the "sustained targeting of multiple entities" over a series of months with millions of machines around the word being targeted.
She said "the United States estimates that (President Bashar) Assad has used chemical weapons in the Syrian war at least 50 times".
Russian officials - both from Moscow and speaking at the United Nations - alleged quickly after Saturday's suspected attack that the images of the victims in Douma were fake.
Western powers are thought to be preparing for strikes but Russian Federation, a Syrian ally, opposes such action.
"Just deterring this act with a few airstrikes and not looking at the consequences for Syria wouldn't be a fully fleshed-out action", said another senior administration official.
On Thursday, "Fox & Friends" host Steve Doocy peddled a false, Russian-backed smear against the Syria Civil Defence (more commonly known as the White Helmets), a group of first responders in Syria who are frequently targeted by Russian disinformation campaigns. This is the second time that the USA has waded into Syria in seven-year conflict.
White Helmets first-responder volunteer and activists claimed an alleged chemical attack on April 7 by the Syrian government killed over 40 people in the town of Douma, the allegations that drew global outrage and prompted Washington and its allies to consider a military response.More news: No US military activity in Syria's Homs at this time - Pentagon
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The US move to punish Russia's economy was a response to an escalation of violence in Syria and the poisoning of a former Russian spy in the UK.
"Eventually, he got exasperated", said a source familiar with the internal debate at the White house.
As worldwide chemical weapons experts travelled to Syria to investigate a suspected gas attack by government forces which killed dozens of people, French President Emmanuel Macron spoke to Russia's Vladimir Putin and expressed concern about a worsening situation.
SIMON: At the same time, there was a lot of concern obviously on behalf of the U.S., Britain and France that Russian Federation, that has military assets on the ground there in Syria, might make some kind of military response. "That said, the president has been consistent and tough on Russian Federation". The hits on several military bases can not, after all, change the facts on the ground-the capture of Eastern Ghouta by al-Assad's troops.
President Vladimir Putin used the pause to engage in some diplomatic exchanges, which had neither the feeling of urgency nor any chance for success.
The Syria crisis has come as a test for not only Trump, but incoming National Security Adviser John Bolton, a hawk who joined Trump's team this week. The ministry additionally pointed to previous alleged use of chemicals by the rebels in fighting with Syrian government troops.
"Again, we are being threatened".
The U.S. ambassador to the United Nations, Nikki Haley, said President Donald Trump "has not yet made a decision about possible actions in Syria".
"Russia vows to shoot down any and all missiles fired at Syria".