Russia blocked an American-sponsored United Nations Security Council resolution on Tuesday that would extend the life of a panel investigating who has committed chemical weapons assaults in the Syria war, in a session punctuated by Cold War-era acrimony.

The outcome undermined one of the few areas of international cooperation in seeking accountability for atrocities committed in the Syria war and cast fresh doubt on whether users of chemical weapons in the conflict will ever be held to account. The use of such weapons is a war crime.

Both sides accused each other of cynicism and dishonesty in acidic exchanges before and after the vote on the resolution, which was vetoed by Russia using its power as a permanent Security Council member.

It was the ninth time Russia had exercised its veto at the Security Council to block a resolution concerning the nearly seven-year-old conflict in Syria. Russia is the Syrian government’s most important ally.