The UN Security Council is set to vote Wednesday on a resolution demanding that the Syrian government cooperate with an investigation of the suspected chemical attack in Idlib province, but Russia is likely to veto the measure, diplomats said.
Britain, France and the United States put forward the draft text which was slightly revised from a proposal presented last week in response to the suspected sarin gas attack in the rebel-held town of Khan Sheikhun.
The vote is scheduled for 3 pm (1900 GMT) and comes as US Secretary of State Rex Tillerson is in Moscow to pressure Russia to reconsider its support for Syrian President Bashar al-Assad.
Russia is expected to use its veto to block the measure, diplomats said. It would be the eighth time that Moscow has resorted to its veto power at the Security Council to block action against its ally Syria.
The draft resolution expresses "horror" at the reported use of chemical weapons in Khan Sheikhun and condemns the alleged attack on April 4.
The text expresses the council's full backing for investigators from the Organization for the Prohibition of Chemical Weapons (OPCW) and the joint UN-OPCW investigative panel tasked with determining who is responsible for such attacks.
It calls on the Syrian government to provide flight plans, flight logs and other information on its military operations on April 4, hand over the names of commanders of any aircraft and provide access to air bases to UN investigators.
The West has accused Assad's forces of carrying out the attack that killed at least 87 civilians, including 31 children, and shocked the world.
On Thursday, the United States fired cruise missiles at a Syrian air base that Washington says was used to launch the chemical attack.
The revised text incorporates suggestions from China, Russia and the other 10 council members.
"We have drafted that resolution deliberately with consensus in mind," British Ambassador Matthew Rycroft told reporters.
"I find it very hard to understand how any member of the Security Council could vote against any part of that resolution."
The Security Council last week discussed three separate draft texts but failed to agree on a way forward and there was no vote on any of the measures.
On Tuesday, Russian President Vladimir Putin called for a thorough investigation by the OPCW and suggested that Assad was the target of false accusations. Syria has denied using chemical weapons.
"It's very important that there is a full investigation so that everybody knows, everybody knows, so that the entire world knows how these horrible chemical attacks occurred and where they come from," French Ambassador Francois Delattre said.
UN investigators already have the mandate under previous resolutions to look into allegations of chemical weapons use but the council wants to underscore its support for a thorough investigation in the Khan Sheikhun case.
Syria is obliged to cooperate with the OPCW as a member of the Chemical Weapons Convention, which it joined in 2013, but access to sites has been a complicated issue.
Turkey's health minister meanwhile confirmed post-mortem tests on victims showed that the deadly nerve agent sarin had been used in Khan Sheikhun.