U.S. officials said they had a "very high level of confidence" that the Syrian government was responsible for the alleged chemical weapons attack that has brought Washington to the verge of a new military intervention.
The comments on April 13, from the White House and the State Department, came as Washington and Moscow sparred at a United Nations Security Council emergency meeting and the United Nations chief called on world powers to "act responsibly."
The United States and its allies are nearing a decision about military action against Syria in response to a recent chemical attack that killed dozens of civilians in Douma. The town was the final holdout in the rebels' former stronghold of eastern Ghouta, outside Damascus.
A second U.S. guided-missile cruiser has moved into the eastern Mediterranean Sea, and jets and other weaponry from Britain and France are also in the region.
An investigative team dispatched by the Organization for the Prohibition of Chemical Weapons was scheduled to arrive in Douma on April 14 to collect evidence.
Russia, which called the UN meeting and is a stalwart backer of President Bashar al-Assad's government, has dismissed the claims, and even asserted that Britain may have been responsible.
Asked to respond, White House spokeswoman Sarah Sanders again laid blame on Moscow.
"We have a very high confidence that Syria was responsible," she said. Russia's failure to stop Syria has been "part of the problem."
At the United Nations, Russian envoy Vasily Nebenzya asserted that the United States, Britain, and France were bent on ousting Assad and containing Russia.
U.S. Ambassador Nikki Haley told the meeting that the U.S. administration estimates that Assad's government used chemical weapons at least 50 times during the seven-year-long conflict.
"Our president has not yet made a decision about possible action in Syria. But should the United States and our allies decide to act in Syria, it will be in defense of a principle on which we all agree," Haley said.
Syria's UN envoy, Bashar Jaafari, vowed that Damascus will have "no other choice" but to defend itself if the West launches military action.
"This is not a threat. This is a promise," he said.
Ahead of the UN meeting, Russian Foreign Minister Sergei Lavrov claimed that the suspected chemical attack was staged last weekend with the aid of an unspecified foreign intelligence agency.
Russian Defense Ministry spokesman Major General Igor Konashenkov said the military had "proof that testifies to the direct participation of Britain in the organizing of this provocation." He did not elaborate or provide evidence.
The accusation was vehemently denied during the emergency meeting by Britain's UN envoy, Karen Pierce, who called it "bizarre" and "a blatant lie."
"This is grotesque," Pierce told reporters as she left the meeting. "It's some of the worst piece of fake news we've yet seen from the Russian propaganda machine."
French Ambassador Francois Delattre said the Assad regime "reached a point of no return. France will shoulder its responsibility to end an intolerable threat to our collective security."
On April 12, France said it had proof the Syrian government had used chemical weapons in the Douma attack, in which the World Health Organization said 43 people who died suffered "symptoms consistent with exposure to highly toxic chemicals."
In his address, UN Secretary-General Antonio Guterres called the situation in the Middle East "chaos" and he said it had become a threat to international peace and security -- and Syria "represents the most serious threat."
The highly volatile situation risks "escalation, fragmentation, and division," he said. "The Cold War is back -- with a vengeance but with a difference," because past safeguards that kept the risk of escalation in check "no longer seem to be present."
Russia has thousands of troops and military advisers in Syria, dozens of aircraft at the Hmeymim air base , and as many as 15 warships off the Syrian coast.
Meanwhile, UN war crimes investigators condemned the suspected use of chemical weapons and said that evidence must be preserved to avoid compromising potential prosecutions.
French President Emmanuel Macron and Russian President Vladimir Putin discussed the situation in Syria during a telephone conversation on April 13, the Elysee Palace said.