Rescue workers have gathered soil samples from the scene of a chemical weapons attack in Syria and sent them to western intelligence officials.
The west is seeking to determine precisely what nerve agent was used in one of the worst atrocities of the country’s six-year war.
The death toll from the attack on the town of Khan Sheikhun in Idlib province has risen to at least 72, with more than 300 other people harmed.
The US, Britain and France said it was very likely that sarin, a nerve agent banned under international regulations, was dropped on the town.
As global condemnation of Syrian president Bashar al-Assad mounted, Russia swung further behind its ally.
It denounced a proposed UN resolution that blamed Assad’s regime for the attack and claimed that a depot used by opposition groups to store a nerve agent had been accidentally bombed.
However, weapons experts laughed that off as “pretty fanciful.”
Nikki Haley, the US ambassador to the UN, chaired a security council emergency meeting that rebuked Moscow for failing to rein in its ally. She asked: “How many more children have to die before Russia cares?”
Haley issued what appeared to be a threat of unilateral action if security council members could not agree on a resolution condemning the attack.
“When the United Nations consistently fails in its duty to act collectively, there are times in the life of states that we are compelled to take our own action,” she said.
Predictably, Russia defended its ally. Its deputy ambassador, Vladimir Safronkov, accused the west of being obsessed with overthrowing Assad instead of seeking peace.
US president Donald Trump described the attack as “horrible”, “unspeakable” and a “terrible affront to humanity”. Asked whether he was formulating a new policy towards Syria, Trump told reporters: “You’ll see.”
British foreign secretary Boris Johnson repeated his claim that the Assad government was responsible and urged the Syrian leader to step down.
“Objectively, I simply don’t see how Bashar al-Assad can remain in charge after what he has already done,” he said.
“Of the 400,000 who are estimated to have been killed in Syria, he is responsible for the vast majority of that butcher’s bill.
“And you have to go a long way back in history to find a tyrant who has stayed in office given such circumstances.”
Witnesses said at least one Syrian warplane dropped bombs on Khan Sheikhun.
Shortly afterwards, first responders were overwhelmed with patients suffering from acute breathing problems, foaming at the mouth and writhing in pain.
Aid agencies said patients showed clear symptoms of exposure to sarin.
Many victims appear to have been poisoned as they slept in their beds.
“The smell reached us here in the centre; it smelled like rotten food,” a nurse told Amnesty International. “We’ve received victims of chlorine attacks before – this was completely different.
“Victims had vomit from the nose and mouth, a dark yellow colour, sometimes turning to brown. Paralysis in respiratory functions – children were dying faster than adults because of this.
“We tried injections … but it just didn’t work. Victims were unable to swallow – they were unconscious, completely unresponsive.”