“All parties share guilt for completely disregarding the rules of war and for failing to adequately protect civilians,” the panel’s chairman, Paulo Sérgio Pinheiro, told reporters in Geneva.

The Russian mission to the United Nations in Geneva did not immediately respond to the findings.

The grim conclusions, based on more than 500 interviews, coincided with reports of heavy civilian casualties in the Damascus suburb of eastern Ghouta, where Syrian and Russian forces pushed on with a ferocious offensive aimed at crushing the last major rebel stronghold in the area.

Airstrikes and shelling there on Monday killed 94 civilians, according to the Union of Medical Care and Relief Organizations, based in France. They also forced an aid convoy, the first to reach the enclave in over three months, to leave before it had fully unloaded.

United Nations investigators had previously reported Russia’s critical role in the war in the two years since it stepped in to rescue of President Bashar al-Assad’s faltering army, but a panel member, Hanny Megally, said this was the first time the commission had been able to nail down Russian involvement in a specific incident.

The panel cited evidence from early warning observers in Syria, who tracked the fixed-wing aircraft from a Russian air base 100 miles from Al Atarib, until its attacks in the early afternoon, when the town’s market was crowded with people who had left work.