Britain and France have called on Russia to restrain Syria's Bashar al Assad after an apparent chemical attack killed at least 73 people.
Damascus has denied it was behind the attack and staunch ally Russia said the gas contamination was the result of a leak from a rebel chemical weapons depot hit by government airstrikes.
But Britain's ambassador to the United Nations echoed remarks by Foreign Secretary Boris Johnson as he said the attack in rebel-held Idlib "bears all the hallmarks" of the Syrian government.
Speaking at an emergency meeting of the UN Security Council, Matthew Rycroft challenged Moscow to stop supporting the regime, which has been fighting a civil war for more than six years.
"What is your plan? What is your plan to stop these horrific senseless attacks? We had a plan and we had the support and you rejected it to protect Assad."
Footage from after the attack in the town of Khan Sheikhoun showed people gasping for air, choking and convulsing in the streets and in overcrowded hospitals.
Others foamed at the mouth as rescue workers hosed down children to wash away any chemicals from their skin and gave them oxygen.
Mr Rycroft accused the Syrians of "a sustained attack using aircraft over a number of hours".
"We see all the signs of an attack using a nerve agent capable of killing over a hundred people and harming hundreds more."
France's UN ambassador accused the Syrian government of "war crimes".
"We urge Russia to exert much stronger pressure to the regime," Francois Delattre said.
"Frankly we also need an America that is seriously committed to a solution in Syria and that puts all its weight behind it."
US President Donald Trump denounced the attack as "terrible affront to humanity" during a meeting with Jordan's King Abdullah.
Asked if he was formulating a new policy towards Syria, Mr Trump said: "You'll see."
Later he said the attack "crossed a lot of lines".
The US ambassador to the UN appeared to threaten possible unilateral action if the Security Council could not agree.
"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," Nikki Haley said.
The UK, France and the US have proposed a UN Security Council resolution to "condemn in the strongest terms the use of chemical weapons".
The draft calls on the Organisation for the Prohibition of Chemical Weapons to quickly reveal its findings on the attack.
It also urges UN Secretary-General Antonio Guterres to report monthly on whether the Syrian government is co-operating with an international inquiry into the use of chemical weapons.
However, as the Security Council began its emergency meeting, there was little hope the proposed resolution would be adopted as Russia called it "unacceptable", based on "fake" information and "anti-Syrian".
It has claimed Syrian aircraft hit a "terrorist warehouse" containing "toxic substances" - but US sources dismissed this explanation.