The report also provided a stark challenge for senior Russian and American officials who are to meet in Geneva on Wednesday to discuss how to bring all the parties together for a peace conference.

In London, the British Foreign Office withheld immediate comment on the report, which recalled a warning by President Obama that the use of chemical weapons in Syria would cross a “red line” even as the United States and Russia work toward convening the conference on Syria.

The United Nations panel, which is seen by diplomats as providing the most factual and authoritative record of developments in Syria, said, “There are reasonable grounds to believe limited quantities of toxic chemicals were used” in Aleppo and Damascus on March 19, in Aleppo again on April 13 and in Idlib on April 29. “Other incidents remain under investigation,” the panel reported.

They based their assertion on interviews with victims of attacks, refugees from Syria and some medical personnel, Mr. Pinheiro, the panel chairman, told reporters on Monday, but he refused to give further details. French authorities have agreed to share with the panel the results of an analysis they are conducting of samples received from casualties who had made their way to Turkey, he said.

Carla Del Ponte, one of the commission members, told Swiss-Italian television last month that testimony from victims pointed to the use of the nerve gas sarin by rebel groups, but other members of the commission quickly distanced themselves from her assertion and their report said they could not identify either the chemical agents used, the means of delivery or the people using them. Use of chemical weapons, they noted, would constitute a war crime.

The especially deadly thermobaric bombs were used in March in the fierce struggle for the strategic town of Qusayr, the panel reported. “If the use was indiscriminate, this could be a war crime,” Mr. Pinheiro said.

The panel cited increasing use of indiscriminate weapons, including cluster munitions, barrel bombs and surface-to-surface missiles as evidence of the government’s “flagrant disregard” for the distinction between combatants and civilians demanded by international law. “There is a strong element of retribution in the government’s approach, with civilians paying a price for ‘allowing’ armed groups to operate within their towns,” the report said.