UNITED NATIONS — Russia and the United States were on a collision course ahead of a UN Security Council vote on Thursday on the fate of a UN-led probe to determine who is behind chemical attacks in Syria’s six-year war.

Washington and Moscow have put forward rival draft resolutions on renewing for a year the mandate of the Joint Investigative Mechanism (JIM), tasked with identifying perpetrators of Syria’s toxic gas attacks.

After negotiations failed to bridge differences, the rivals each called for a council vote on their draft resolutions on Thursday, hours before the JIM’s mandate expires at midnight.

Diplomats said they expected Russia to veto the US-drafted measure, which would be the 10th time Moscow has used its veto power at the council to block action targeting its Syrian ally.

The Russian text is unlikely to garner the nine votes required for adoption, diplomats said.

A resolution requires nine votes to be adopted at the council, but five countries -- Russia, Britain, China, France and the US -- can block adoption with their veto power.

"The US 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."

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 US to launch missile strikes on a Syrian air base a few days later.

Washington and its allies have blamed President Bashar al-Assad’s government for the Khan Sheikhun attack, but Syria has denied using chemical weapons, with strong backing from Russia.

Victory for those who use chemical weapons

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 be extended for a year.

During a council vote in late October, Russia vetoed a US-drafted resolution on a one-year extension, arguing that it did not want to decide on the fate of the panel before the Khan Sheikhun report.

The US, Britain and France have insisted that the JIM should be allowed to continue its work and that dozens of other cases of chemical weapons use in Syria must be investigated.

Britain said ending the investigation would mean that perpetrators of chemical weapons attacks in Syria will go unpunished.

"The only victors would be people who want to use chemical weapons in Syria, which is the Assad regime plus Daesh, and I think everyone in the Security Council would be shooting ourselves in the foot if we allowed that to happen," said British Ambassador Matthew Rycroft.

Daesh is an Arabic-derived acronym that refers to the Islamic State group.

Russian Ambassador Vassily Nebenzia said this week that scrapping the chemical weapons probe in Syria "may send a bad signal, but the way the investigation has been conducted sends an even worse signal."

The joint UN-Organisation for the Prohibition of Chemical Weapons (OPCW) panel was set up by Russia and the US in 2015 and unanimously endorsed by the council, which renewed its mandate last year.

Previous reports by the JIM have found that Syrian government forces were responsible for chlorine attacks on three villages in 2014 and 2015, and that the Islamic State group used mustard gas in 2015. — AFP