The UN Security Council is expected to adopt on Friday a draft resolution that would set up a panel to determine who is responsible for deadly chlorine gas attacks in Syria, diplomats said.
A vote at the 15-member council was scheduled after US Secretary of State John Kerry won backing from Russia for the measure, in a rare sign of cooperation from the Damascus ally over how to address the conflict in Syria.
Under discussion for months, the US-drafted resolution would set up a team of experts tasked with identifying the perpetrators of the chemical weapons attacks, paving the way for possible sanctions.
The resolution mandates the panel jointly set up with the OPCW chemical weapons watchdog to "identify to the greatest extent feasible individuals, entities, groups, or governments who were perpetrators, organisers, sponsors or otherwise involved in the use of chemicals as weapons including chlorine or any other toxic chemical" in Syria.
The United States and its allies have repeatedly accused President Bashar al-Assad's forces of carrying out chlorine gas attacks with barrel bombs thrown from helicopters.
The three countries argue that only the Syrian regime has helicopters, but Russia maintains there is no solid proof that Damascus is behind the attacks.
"So what we are trying to do is to get beyond the mere finding of the fact that it may have been used, and actually find out who used it, and designate accountability for its use," Kerry told reporters in Malaysia earlier.
Kerry said he agreed with Russian Foreign Minister Sergei Lavrov on the measure during a meeting on Wednesday.
The measure would task UN Secretary-General Ban Ki-moon to set up the team within 20 days, working with the Hague-based Organization for the Prohibition of Chemical Weapons (OPCW).
The panel would present its first findings to the council 90 days after it begins its work.
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Pressure has been mounting on the deeply-divided Security Council to take action in Syria, where the war is now in its fifth year and has claimed more than 230,000 lives.
- Russian shift -
A veto-wielding member of the council, Russia last year blocked a key resolution on referring Syria to the International Criminal Court for war crimes but it later backed a measure on boosting humanitarian aid.
Russian support for the chlorine gas probe is seen by some western diplomats as a shift from Moscow, which has shielded the Assad regime at the United Nations.
"There is a change of tone," a Security Council diplomat said this week, but he cautioned: "I don't want to overstate it."
Security Council diplomats are separately working on a statement backing a new push for peace talks in Geneva that could yield a plan for a Syrian transition that western powers insist should happen without Assad.
Discussions are also inching forward on a new tougher UN measure on banning the use of barrel bombs, building on resolutions that have condemned the practice.
In 2013, Syria agreed to a US-Russia plan to dismantle its chemical weapons network and join an international treaty banning their use in what was then hailed as a first sign that Moscow was ready to turn up the pressure on Assad.
But human rights groups and Syrian doctors have since come forward with videos and accounts of dozens of chlorine gas attacks that have in particular targeted the northwestern Idlib province.
Earlier this year, council members heard graphic accounts from Syrian doctors of chlorine gas attacks in March on the village of Sarmin that left six dead including three children.