UNITED NATIONS (AP) — The United States is calling for a vote Thursday on a resolution to extend the mandate of experts trying to determine who was responsible for chemical attacks in Syria, a measure that is likely to face a Russian veto.

A rival Russian resolution, which is opposed by the U.S. and other Security Council members, is also expected to be put to a vote.

The result is likely to be that neither resolution is adopted and the Joint Investigative Mechanism, known as the JIM, will cease operations when its current mandate expires at midnight Thursday.

This would be a serious blow to efforts to hold those responsible for chemical weapons attacks in Syria accountable.

Louis Charbonneau, U.N. director at Human Rights Watch, warned last year that "without accountability, the cycle of abuses in Syria by all parties to the conflict — whether by chemical or conventional weapons — will continue unabated."

At the heart of the dispute is the demand by Russia, Syria's most important ally, for changes in the way the JIM operates.

In September 2013, Syria accepted a Russian proposal to relinquish its chemical weapons stockpile and join the Chemical Weapons Convention. That averted a U.S. military strike in response to an alleged chemical weapons attack that killed hundreds in the Damascus suburb of Ghouta.

The Organization for the Prohibition of Chemical Weapons, which implements the 1997 convention, investigates alleged use of chemical weapons. But it is not authorized to determine responsibility and that is why the United States and Russia worked together to draft the resolution establishing the U.N.-OPCW Joint Investigative Mechanism in 2015.

But Russia has been highly critical of the JIM's findings that the Syrian government used chlorine gas in at least two attacks in 2014 and 2015 and used sarin in an aerial attack on Khan Sheikhoun last April 4 that killed about 100 people and affected about 200 others who survived the nerve agent.

Syria has denied any use of chemical weapons, while Russia has accused the JIM of using faulty methods to determine that President Bashar Assad's government was to blame for the attacks, including not visiting Khan Sheikhoun.

The JIM has also accused the Islamic State extremist group of using mustard gas in 2015 and again in September 2016 in Um Hosh in Aleppo.

The U.S. Mission to the United Nations said in a statement Wednesday calling for the vote that "there are many more instances of chemical weapons use in Syria that must be investigated."

"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," the statement said. "Not doing so would only give consent to such atrocities while tragically failing the Syrian people who have suffered from these despicable acts."

Both the U.S. and Russian draft resolutions would extend the JIM's mandate that the council agreed to in previous resolutions for another year. They say the JIM should work "in an independent, impartial and professional manner in implementing its mandate."

But the U.S. draft would support the JIM's experts pursuing investigations "in a manner they deem appropriate." And it would underscore the importance of the JIM "conducting its investigations according to high methodological standards" and basing its findings on "evidentiary levels" outlined in 2016.

The Russian draft, by contrast, requests the JIM to visit Khan Sheikhoun and re-evaluate its findings. And it welcomes the "full and profound cooperation" extended by the Syrian government in establishing facts related to the Khan Sheikhoun and Um Hosh attacks.

It also would order the JIM to be guided by the "high standards" in the Chemical Weapons Convention and using additional evidence in its investigations. And it would ask Secretary-General Antonio Guterres to recruit experienced staff from as wide a geographical basis "as practicable."