As of Friday it remained unclear when the Security Council vote would be scheduled, but Western diplomats hoped it could happen early next week. A Russian veto, should there be one, would put Russia in the position of shutting down what has been one of the few areas of cooperation with the United States and its allies on accountability in the war in Syria.

The deployment of chemical weapons, a war crime, has been a recurrent theme of the atrocities committed during the Syrian civil war, which has dragged on for more than six years.

The panel seeking to identify who committed the chemical attacks, known as the Joint Investigative Mechanism, was created a few years ago as a collaborative operation of the United Nations and the Organization for the Prohibition of Chemical Weapons, the group based in The Hague that monitors the global ban on such munitions.

Panel investigators have since found that government forces in Syria used chlorine-filled bombs on at least three occasions on 2014 and 2015 against insurgent-held areas, and Islamic State militants used mustard poison at least once. But the Khan Sheikhoun inquiry has been the panel’s biggest single undertaking.

The attack on Khan Sheikhoun led President Trump to order a punishing missile strike a few days later on the Shayrat military airfield in Syria where, American intelligence officials said, aircraft carrying the poison weapon had originated.

Russia accused the United States of rushing to judgment on who was responsible and initially asserted that the sarin attack had been fabricated as a pretext to vilify Mr. Assad. It strongly condemned the missile strike and threatened to disband the military communications that have minimized the risk of conflict between Russian and American forces operating in Syria.

The Russians have always expressed skepticism about both the Joint Investigative Mechanism’s objectivity and its ability to collect credible evidence in a war zone.