Russia used its veto power Friday to block a stopgap measure at the United Nations Security Council that would have temporarily preserved the panel investigating who is using chemical weapons in the Syrian conflict.

It was the second Russian veto in 24 hours aimed at ending the panel, which has found since its creation by the Security Council two years ago that both the Syrian government and Islamic State militants have used chemical weapons in the conflict, a war crime.

The panel, known as the Joint Investigative Mechanism or J.I.M., needed reauthorization from the Security Council by Friday to continue its work.

Russia, the Syrian government’s most important ally, has now used its veto 11 times to shield the government of President Bashar al-Assad of Syria since the war that has convulsed that country began in 2011.