With possible termination looming for a United Nations panel responsible for identifying chemical weapons users in the Syria war, Russia signaled Thursday that it might veto an American-drafted Security Council resolution extending the panel’s life for another year.

The Russians’ stance, conveyed by Foreign Minister Sergey V. Lavrov in Moscow, came hours before a scheduled vote at the 15-member Security Council on the American resolution, which diplomats said was supported by most members.

Russia, the Syrian government’s most important ally, was infuriated by the panel’s Oct. 27 report, which found that the Syrian military had been responsible for a deadly sarin attack on the village of Khan Sheikhoun in northern Syria on April 4.

Panel investigators relied on interviews, photos, videos and analysis of soil samples supplied by the Syrian government from Khan Sheikhoun. But they did not go to the attack site.

Calling the report deeply flawed and tainted by Western political pressure to vilify the Syrian government, the Russians rejected the findings and suggested that Syrian insurgents or Islamic militants might have staged the attack.

While the Russians expressed support for renewing the panel’s mandate, they also demanded changes in its investigation methods.

The United States and its allies have strongly defended the panel’s integrity and professionalism and have accused Russia of attempting to neuter the panel’s ability to identify who is behind the chemical weapons attacks in Syria, which are a war crime.

Speaking at a news conference in Moscow on Thursday, Mr. Lavrov rejected the Western accusations and countered that in Russia’s view, the United States and its allies had attempted to evade the truth.

Mr. Lavrov said the panel’s investigators had shown themselves to be “biased, politicized, clearly carrying out orders from the outside.”

The American resolution, he said, “aims to extend the mandate of this mechanism by welcoming and approving the methods that it uses.”

“It is clear that this is absolutely unacceptable,” Mr. Lavrov said.

If Russia’s delegation at the United Nations blocks the resolution, it would be the 10th time that Russia has used its veto power to shield the government of President Bashar al-Assad of Syria from Security Council action since the war began nearly seven years ago.

The chemical weapons panel, known as the Joint Investigative Mechanism or J.I.M., is a collaboration of the United Nations and the Organization for the Prohibition of Chemical Weapons, which monitors compliance with the treaty banning them.

The panel requires a new Security Council mandate to continue working beyond 11:59 p.m. Thursday, when the current one expires.

The panel’s leader, Edmond Mulet, a veteran United Nations diplomat, said in an interview last week that it had been forced to wind down investigations of other possible chemical attacks in Syria because of the uncertainty.

Western diplomats have privately expressed anger at what they called Russian intransigence despite attempts to compromise on a resolution to extend the panel’s mandate. The American resolution, which originally proposed a 24-month renewal of the mandate, was weakened to 18 months as part of an attempt to appease Russia. Now it is down to 12 months.

Dissolution of the panel threatened to nullify an important area of cooperation between Russia and the United States in the Syria war, despite their bitter differences over who is to blame in the at conflict.

Chemical weapons disarmament experts, who welcomed the creation of the investigative panel two years ago, o expressed alarm about its possible demise, saying that it would send a signal to chemical weapons users that they could act with impunity.

“It is very important that the J.I.M. investigations go forward,” said Paul F. Walker, director of environmental security and sustainability at Green Cross International, a disarmament advocacy group. Another Russian veto, he said, would bring “serious doubt to Russia’s verbal support of a world free of chemical weapons.”