The United States claimed Friday to have proof that the Syrian regime carried out a chemical massacre that has set western powers on a collision course with Russia, as President Donald Trump readied his response.
Western leaders have made clear the alleged toxic gas strike on the Damascus suburb of Douma must not go unanswered -- despite a warning from the UN secretary-general that the crisis could trigger a "full-blown military escalation."
Trump vowed a "strong" response the day after the April 7 attack -- since then, he has been in consultation with his military advisors, and allies France and Britain, on what form that should take.
British Prime Minister Theresa May spoke with French President Emmanuel Macron late Friday and agreed to "keep working closely" on a response to the attack, which the British government now believes killed as many as 75 people.
With experts from the Organisation for the Prohibition of Chemical Weapons (OPCW) headed to Syria to probe the strike, Russia defiantly claimed Friday the attack was staged by rescue workers acting on Britain's behalf.
Western powers have been increasingly categorical in laying responsibility with Syria's Bashar al-Assad -- and Moscow.
Asked whether Washington has proof Assad launched the attack on Douma, the main city in the former rebel bastion of Eastern Ghouta, State Department spokeswoman Heather Nauert said: "Yes."
"We know for a fact that it was a chemical weapon," she said, but added that she would not be able to release US "intelligence information" as it was still classified.
Trump's spokeswoman Sarah Sanders said Washington also holds Assad's ally "Russia responsible for their failure to stop chemical weapons attacks from taking place."
- Military escalation -
Trump's National Security Council was meeting Friday evening at the level of agency deputy heads, she said.
Any intervention would increase the risk of a clash with Russian forces in Syria to defend Assad, and UN chief Antonio Guterres warned the UN Security Council -- where the US and Russia faced off -- to beware a "full-blown military escalation."
France's Macron earlier called for stepped-up talks with Moscow, and spoke to President Vladimir Putin by phone.
But France's UN ambassador Francois Delattre told the Security Council that in choosing once again to use banned chemical weapons against civilians, Assad's regime had "reached a point of no-return."
And US ambassador Nikki Haley, while allowing that Washington is still weighing its options and pursuing its own investigation, warned her colleagues, "At one point, you have to do something."
"All nations and all people will be harmed if we allow Assad to normalize the use of chemical weapons," she said.
Russia, in addition to its unproven allegations against Britain, alleged the West was feigning outrage over the attack as a cover for a plan to overthrow Assad's government.
"We continue to observe dangerous military preparations for an illegal act of force against a sovereign state," Russian Ambassador Vassily Nebenzia told the Security Council.
Rebels give up Ghouta
Since last weekend, when images of ashen toddlers struggling for breath emerged, there has been a sustained military buildup in the eastern Mediterranean.
A French frigate, British Royal Navy submarines and the USS Donald Cook, an American destroyer equipped with Tomahawk land attack missiles, have all moved into range of Syria's coast.
On the ground, rebels and civilians were evacuating from Douma on Friday after anti-regime fighters in Eastern Ghouta surrendered their heavy weapons and their leader left the enclave.
This signaled the end of one of the bloodiest assaults of the seven-year war and a major win for the Assad regime.
Western officials believe chlorine was used in the April 7 attack on Douma.
What is less clear is whether sarin, the agent used in a chemical attack that prompted US missile strikes last year, was also used.
Russia, which has stonewalled diplomatic efforts at the UN Security Council, has vehemently denied a chemical attack took place.
OPCW inspectors are expected to arrive in Syria at the weekend to investigate, following an invitation from Damascus.
Diplomats have expressed concern that the experts could be used as hostages or human shields.