US Ambassador Nikki Haley warned on Wednesday that US President's Trump administration will take action against chemical attacks in Syria - if the UN Security Council fails to act.
Haley - who said the attack bears "all the hallmarks" of President Bashar Assad's government - urged the council at an emergency meeting to immediately approve a resolution drafted by the US, Britain and France that condemns and threatens consequences for the use of chemical weapons.
Tuesday's attack in rebel-held Idlib province killed dozens of people.
"There are times at the United Nations when we are compelled to take collective action," she said. "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."
"For the sake of the victims, I hope the rest of the council is finally willing to do the same," she added.
Haley spoke after Russian Foreign Ministry spokeswoman Maria Zakharova announced Moscow's opposition to the draft resolution.
She called it "categorically unacceptable" because "it runs ahead of the investigation results and names the culprit, Damascus."
"The main task now is to have an objective inquiry into what happened," Russia's deputy UN ambassador Vladimir Safronkov told the Security Council. "Up to now all falsified reports about this incident have come from the White Helmets or the Syrian Observatory for Human Rights based in London which have been discredited long ago."
He said that from 11:30am until 12:30pm on April 4, the Syrian government carried out an airstrike on the eastern edge of the opposition-held town of Khan Sheikhoun "on a large warehouse of ammunition and military equipment".
On that compound, he said, there was a facility "to produce ammunition with the use of toxic weapons" which was supposed to be used in Iraq and Aleppo.
"Their use was confirmed last year by Russia and military experts," Safronkov said. "The symptoms of those affected in Khan Sheikhoun were the same as those by people who were affected last year in Aleppo."
Britain's UN Ambassador Matthew Rycroft also told the council that Tuesday's attack "bears all the hallmarks" of President Bashar Assad's regime.
"We have every indication that this was a sustained attack using aircraft over a number of hours," Rycroft said. "We see all the signs of an attack using a nerve agent capable of killing over a hundred people and harming hundreds more."
The resolution drafted by the US, Britain and France would condemn the use of chemical weapons in Syria, especially on Tuesday, "in the strongest terms" and back an investigation by the international chemical weapons watchdog, the Organization for the Prohibition of Chemical Weapons.
It stresses that the Syrian government, under previous Security Council resolutions, is obligated to provide OPCW investigators and a UN-OPCW team charged with assessing blame for chemical attacks with information on all flight plans and air operations on April 4 and the names of commanders of any helicopter squadrons.
Syria's deputy UN ambassador Mounzer Mounzer told the council his government categorically rejects "false claims and accusations" that the army used toxic chemicals against Syrian civilians in Khan Sheikhoun, saying they are being used "as human shields by armed terrorist groups".
He said the army does not have any type of chemical weapons and "we have never used them and we will never use them."
The death toll from Tuesday's attack has risen to 86, the UK-based Syrian Observatory for Human Rights said.