WASHINGTON — President Donald Trump is pressing for the United Nations Security Council to extend the mandate of experts trying to determine who was responsible for chemical attacks in Syria.

The measure could face a veto from Russia, which is pushing a rival resolution opposed by the U.S. and other Security Council members.

If neither resolution is adopted, the Joint Investigative Mechanism would cease operations when its current mandate expires at midnight Thursday.

Trump says on Twitter he needs “all” on the council to renew the so-called JIM “to ensure that Assad Regime does not commit mass murder with chemical weapons ever again.”

Russia, Syria’s most important ally, has been seeking changes in the way the JIM operates. Syria accepted a Russian proposal in 2013 to relinquish its chemical weapons stockpile.