The Commission of Inquiry investigating Syria's civil war said it "documents for the first time the systematic imposition of sieges, the use of chemical agents and forcible displacement" in a report released Tuesday, The New York Times reported.
"War crimes, crimes against humanity and gross human rights violations continue apace," said the report, submitted to the U.N. Human Rights Council in Geneva, reporting 17 incidents that could be considered massacres from mid-January to mid-May.
The report also included President Obama's warning that using chemical weapons in Syria would cross a "red line" while the United States and Russia work toward convening an international peace conference on Syria.
Discussions between Europe and the United States about the possibility of supplying weapons to the rebels seeking the overthrow of President Bashar Assad have become more discordant. Last month, Britain and France pressed their European Union partners to allow an arms embargo to Syria to lapse, potentially allowing EU governments to arm rebels, while Russia said it will supply government forces with ground-to-air missiles.
The U.N. panel said it found "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 four-member panel said.
Their comments were based on interviews with victims of attacks, refugees from Syria and medical personnel, panel Chairman Paulo Pinheiro said Monday.
The panel also said government forces showed a "flagrant disregard" for the distinction between combatants and civilians as demanded by international law, the Times reported.
"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.
The siege mentality adopted by government and rebel forces have trapped civilians in their homes, and cut off supplies of food, water, medicines and electricity, the report said, noting the actions clearly breach international law.
The panel also reported instances in which both sides used attacks or the threat of attacks to drive civilians out of particular areas.
While there was a disparity between abuses and crimes committed by government forces and rebel groups, Pinheiro said, "but this is a disparity in intensity. It is not a disparity in the nature of the crimes."