Britain, France and the United States have presented a draft resolution demanding a full investigation of the attack on a rebel-held town in Idlib province, but Russia said the text was 'categorically unacceptable'.
"We are talking about war crimes here, war crimes on a large scale, war crimes with chemical weapons," French Ambassador Francois Delattre told reporters as he entered the council chamber.
The draft resolution calls for a full investigation by the Organisation of the Prohibition of Chemical Weapons (OPCW) of the attack in the early hours of Tuesday in the town of Khan Sheikhun.
At least 72 people, among them 20 children, were killed in the strike, and dozens more were left gasping for air, convulsing, and foaming at the mouth, doctors said.
The proposed measure requests that the joint UN-OPCW investigative panel set up to determine who is responsible for chemical attacks in Syria begin work immediately to identify the perpetrators of the latest attack.
The text, seen by AFP, calls on Syria to provide flight plans, flight logs and other information on its military operations on the day of the assault.
Damascus would be asked to provide the names of all commanders of helicopter squadrons to UN investigators and allow them to meet with generals and other high-ranking officials within five days of their request, the draft resolution said.
Syria would also allow UN and OPCW teams to visit air bases from which the attacks involving chemical weapons may have been launched, according to the text.
No vote was immediately scheduled, but Western diplomats said they would not negotiate for long, suggesting Russia could resort to a veto to block the measure.
"A Russian veto would mean that they are spending more time defending the indefensible," British Ambassador Matthew Rycroft told reporters.
Britain, France and the United States blame President Bashar al-Assad's forces for the attack, but the Syrian army has denied any involvement.
Moscow, which launched a military intervention in 2015 in support of Assad's forces, earlier defended Damascus against accusations of responsibility for the attack.
It said the deaths were caused when a Syrian air strike hit a "terrorist warehouse" used for making bombs containing "toxic substances", and pledged to continue its military support for Assad.
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