The United States and Russia, Syria's ally, have put forward rival draft resolutions on renewing the mandate of the Joint Investigative Mechanism (JIM), tasked with investigating Syria's toxic gas attacks.
The UN Security Council will vote on Thursday on a US-drafted resolution that would extend for a year an international probe to determine who is behind chemical attacks in Syria's six-year war.
The United States and Russia, Syria's ally, have put forward rival draft resolutions on renewing the mandate of the Joint Investigative Mechanism (JIM), tasked with investigating Syria's toxic gas attacks.
After nearly two weeks of negotiations, the two sides failed to bridge differences and the United States called for a vote on its draft resolution. Diplomats said Russia was expected to also request a vote.
"The United States hopes the Security Council will stand united in the face of chemical weapons use against civilians and extend the work of this critical group," said the US mission in a statement.
"Not doing so would only give consent to such atrocities while tragically failing the Syrian people who have suffered from these despicable acts."
The vote is scheduled for 2000 GMT, just hours before the mandate of the so-called JIM expires at midnight.
It remained unclear whether Russia would veto the US- drafted measure, which would be the 10th time that Moscow has used its veto power at the council to block action targeting its Syrian ally.
Russia has sharply criticised the JIM after its latest report blamed the Syrian air force for a sarin gas attack on the opposition-held village of Khan Sheikhun that left scores dead.
The attack on April 4 triggered global outrage as images of dying children were shown worldwide, prompting the United States to launch a missile strike on a Syrian air base a few days later.
Washington and its allies have blamed President Bashar al-Assad's government for the attack, but Syria has denied using chemical weapons, with strong backing from Russia.
In its draft, Russia insisted that the panel's findings on Khan Sheikhun be put aside to allow for another "full-scale and high-quality investigation" by the JIM, which would also be extended for a year.
During a council vote in late October, Russia vetoed a US-drafted resolution that would have extended the mandate of the panel for a year.
Russia then opposed renewing the mandate of the panel before the release of the Khan Sheikhun report.
The joint UN-Organization for the Prohibition of Chemical Weapons (OPCW) panel was set up by Russia and the United States in 2015 and unanimously endorsed by the council, which renewed its mandate last year.
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