UNITED NATIONS — Infuriating the United States and its Western allies, Russia on Thursday blocked an American-sponsored resolution at the Security Council to extend the life of a panel investigating who is using chemical weapons in the Syria conflict.

The veto means the panel, which has found that both the Syrian government and Islamic State militants have used chemical poisons in the war, will be dissolved as of Friday.

Ambassador Nikki R. Haley of the United States denounced the Russian veto, saying “it strikes a deep blow.”

It was the 10th time Russia had used its veto power at the Security Council to protect the Syrian government, its principal ally in the Middle East. The veto threw into doubt whether users of chemical weapons in Syria will ever be held to account.

The use of chemical weapons is a war crime.

Hours before Ambassador Vassily Nebenzia of Russia cast his no vote at a Security Council meeting, Russia’s foreign minister, Sergey V. Lavrov, had signaled at a news conference in Moscow his country’s intention to block the American resolution.

Russia had been angered 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 had expressed support for renewing the panel’s mandate, they also demanded changes in its investigation methods.

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

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.

The chemical weapons panel, known as the Joint Investigative Mechanism or J.I.M., was 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 required 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. How, or even whether, the perpetrators of those attacks will be identified is now unclear.

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

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

Chemical weapons disarmament experts, who welcomed the creation of the investigative panel two years ago, also expressed alarm about its demise, saying that it sends 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. The Russian veto, he said, brings “serious doubt to Russia’s verbal support of a world free of chemical weapons.”