The United States is calling for a vote Thursday on a resolution that would extend the mandate of experts working to determine who was responsible for chemical attacks in Syria, which is likely to face Russian opposition.

A rival Russian resolution 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 will cease operations when its mandate expires at midnight Thursday. This would be a blow to efforts to hold those responsible for chemical weapons attacks in Syria accountable.

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