Western powers said an alleged Syrian chemical massacre must not go unanswered Friday even as the United Nations secretary-general warned against a "full-blown military escalation."
Russia warned the West not to make any "dangerous" moves against its ally Bashar al-Assad's regime and claimed to have proof that the attack had been staged by rescue workers acting on Britain's behalf.
Experts from the Organisation for the Prohibition of Chemical Weapons (OPCW) are on their way to Syria to probe the alleged chlorine strike on the then rebel-held suburb of Douma, which took place on April 7.
But, with the British government now estimating that the death toll from the attack has risen to 75, US President Donald Trump and British and French leaders are contemplating punitive military action.
Asked whether the United States has proof Assad launched the Douma attack, 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."
Trump's National Security Council was to meet later Friday at the level of agency deputy heads, she said.
Any intervention increases the risk of a clash with Russian forces in Syria to defend Assad, and UN chief Antonio Guterres warned the Security Council to beware a "full-blown military escalation."
But France's UN ambassador Francois Delattre told the 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.
Tension between Russia and the West came amid signs the parallel conflict between Israel and Iran's forces and proxies in Syria is also on the point of escalation.
Earlier this week, Israel bombed a Syrian airbase used by Assad's regional allies, killing at least seven Iranians, and prompting threats from Tehran and its Lebanese militia ally Hezbollah.
"The Israelis carried out a historic mistake ... and put themselves in direct combat with Iran," Hezbollah leader Hassan Nasrallah warned on Friday in a televised address.
But the growing fears of escalation appear to have given Western leaders food for thought.
After warning in a belligerent tweet earlier this week that "missiles are coming," Trump backed off from talk of imminent action and met with commanders to look at options.
France's President Emmanuel Macron, meanwhile, on Friday called for stepped-up talks with Moscow and spoke to President Vladimir Putin by phone.
"The most important thing is to refrain from ill-considered and dangerous actions that would constitute a gross violation of the UN Charter and would have unpredictable consequences," the Kremlin said.
Western officials believe chlorine was used in the April 7 attack on Douma, the main city in the former rebel bastion of Eastern Ghouta.
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.
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.