Ms. Haley said there was “overwhelming support” in the Security Council for the panel’s work, which she called essential to finding and prosecuting the perpetrators of chemical weapons attacks in Syria — whoever they may be. “We can’t pick and choose who we want to be at fault and who we don’t,” she said.
France threw its weight behind the Americans on Thursday. “We cannot accept that the credibility and independence of these mechanisms are challenged on the grounds that their conclusions are not suitable for Russia,” a Foreign Ministry spokeswoman, Agnès Romatet-Espagne, told reporters in Paris.
The United Nations secretary general, António Guterres, said in a press briefing on Wednesday that the panel was a “very important tool, a tool addressing the problems of accountability,” and that “we fully support their activities.”
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 two occasions on 2014 and 2015 against insurgent-held areas, and rebels 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.
While the Russians did not object to the creation of the Joint Investigative Mechanism in 2015, they have always expressed skepticism about both its objectivity and its ability to collect credible evidence in a war zone.
Those doubts intensified publicly last Friday, when a Russian Foreign Ministry disarmament expert, Mikhail Ulyanov, spoke at briefing for United Nations member states. He challenged the basic assumption about the Khan Sheikhoun attack that it had been carried out from the air.
Russia’s own photographic analysis, Mr. Ulyanov said, suggested that the crater left by the bomb showed it had been detonated on the ground, calling into question how government forces could have placed it there.
Mr. Ulyanov also said photographs of child victims — which Mr. Trump cited when he ordered the retaliatory attack — suggested that they had been given drugs to feign attack symptoms in what might have been a “staged incident” that the panel needed to address in its findings.
The panel’s chairman, Edmond Mulet, a longtime United Nations diplomat, has not commented on the conclusions of the report ahead of its release. But he is known to be frustrated over what he has viewed as a lack of Syrian cooperation with the investigation.
After a private briefing to the Security Council in July — three months after the Khan Sheikhoun attack — Mr. Mulet told reporters that he needed Syria’s help and that ‘hopefully we’ll be given the necessary tools to do our work.”
The panel’s investigators were never able to visit Khan Sheikhoun because of security dangers, relying instead on other information, including witness testimony, aerial surveillance and soil samples they did not collect themselves.
Both Russia and Syria say the panel’s inability to travel safely within Syria raises doubts about the credibility of the findings. Outside analysts said the argument had some merit — even if the Syrian government’s lack of cooperation was also to blame.
“It’s definitely the weakest point in that investigation and previous investigations that they haven’t been able to get access to the sites,” said Aron Lund, a fellow at The Century Foundation, a research group, who has written extensively about Syria.
For the Khan Sheikhoun investigators, Mr. Lund said, “that is a big issue, and that’s why they might wind up saying, ‘We don’t know.’ ”