The Syrian Air Force was responsible for a lethal sarin chemical attack on a northern rebel-held village on April 4, a United Nations investigative panel said Thursday.

Its report represented the first time that a politically independent investigation had concluded which side in the Syrian civil war had carried out the attack on the village, Khan Sheikhoun.

The early morning attack killed dozens of civilians, including children, and is regarded as one of the worst chemical weapons atrocities in the conflict.

The panel’s finding could place new pressure on Russia, the Syrian government’s most important ally, which pledged four years ago to ensure that President Bashar al-Assad purge all his chemical weapons.

But the determination is also likely to be assailed by Russia as flawed, and could raise doubts about whether the panel, known as the Joint Investigative Mechanism, will be able to continue its work.

A second finding in the report concluded that Islamic State militants had carried out an attack using sulfur mustard poison in Um-Housh, in Aleppo Province, on Sept. 16, 2016.

The use of chemical weapons, a recurrent theme in the conflict that began nearly seven years ago, is a war crime.

The investigative panel, created in 2015 by the United Nations Security Council to determine who has been using chemical weapons in Syria, is a collaboration of the United Nations and the Organization for the Prohibition of Chemical Weapons, The Hague-based group that polices the global ban on such munitions.

The panel’s latest report, which was shared with Security Council members and seen by The New York Times, said it was “confident that the Syrian Arab Republic is responsible for the release of sarin at Khan Sheikhoun on 4 April 2017.”

Its previous reports have found that Syrian forces had used chlorine bombs at least three times in the conflict and that Islamic State fighters had used sulfur mustard at least once. But its investigation into who committed the Khan Sheikhoun assault was regarded as its most important undertaking yet.

It also was seen as a barometer of whether Russia and the West, which support opposing sides in the Syrian war, could continue cooperating on efforts to hold chemical weapons users in Syria accountable.

Western nations immediately seized on the panel’s conclusion as evidence of duplicity by Syria and Russia.

“Time and again, we see independent confirmation of chemical weapons use by the Assad regime. And in spite of these independent reports, we still see some countries trying to protect the regime,” Nikki R. Haley, the United States ambassador to the United Nations, said in a statement. “That must end now.”

Foreign Secretary Boris Johnson of Britain said the Khan Sheikhoun findings represented “an appalling breach of the rules of war,” and he called on “the international community to unite to hold Assad’s regime accountable.”

The United States quickly concluded after the Khan Sheikhoun assault that the Syrian Air Force had been responsible, based partly on what the Americans said was intelligence that traced the attack to aircraft based at Al Shayrat, a Syrian military airfield.

Two days after the sarin assault, President Trump authorized a Tomahawk missile strike on Al Shayrat, enraging Russia and Syria. It was the first direct American assault on a Syrian military target in the conflict.

Russia had signaled before the Khan Sheikhoun report was finished that it would dispute any conclusion of Syrian government responsibility because of what Russia’s own experts called serious gaps in the panel’s investigation.

The Russian experts expressed concern that investigators had not visited Khan Sheikhoun, and said their own surveillance photographs of the bomb site suggested that the crater was too small for the bomb to have been dropped from the air.

With the panel’s mandate due to expire on Nov. 16, the Security Council must decide whether to renew it in the coming days.

An American-led effort at the Security Council to renew the mandate for one year was blocked by a Russian veto on Tuesday. The Russians had said they wanted to see the results of the Khan Sheikhoun investigation first.

Whether Russia will now oppose the renewal is not yet clear.

The report’s conclusion on Khan Sheikhoun was not entirely unexpected because of other evidence that came to light immediately after the attack, including videos, suggesting an aircraft had dropped a bomb containing sarin on a street in the village.

Only Syrian aircraft were known to be operating in that area, and the insurgents have no planes.

Both Syria and Russia initially called the attack a fabrication, and Mr. Assad went so far as to speculate that child actors faking asphyxiation had been used in an elaborate deception by his enemies to portray him as a war criminal.

But it quickly became clear from verified soil and blood samples of victims that sarin, a lethal nerve agent once stockpiled by Mr. Assad’s military, had been used.

Syria and Russia then suggested that insurgents or other nonstate actors in Khan Sheikhoun had planted a sarin bomb on the ground.

The panel said in its report that such a possibility had been explored but rejected, partly because of the large number of people affected and the presence of sarin for days after the attack, “which is consistent with it being dispersed via a chemical aerial bomb.”