UNITED NATIONS (AP) — Rival U.S. and Russian resolutions to extend the mandate of experts trying to determine who was responsible for chemical attacks in Syria were defeated Thursday at a heated Security Council meeting that reflected the deteriorating relations between Washington and Moscow.

The result of the two votes means that the expert body — the Joint Investigative Mechanism known as the JIM — will cease operations when its current mandate expires at midnight Thursday.

The U.S., its allies and human rights groups called it a serious blow to efforts to hold accountable those responsible...