GENEVA (Reuters) -- The Syrian government is reportedly using local militias known as Popular Committees to commit mass killings which are at times sectarian in nature, UN human rights investigators said on Monday.

The uprising in Syria erupted two years ago with largely peaceful protests but escalated into a civil war pitting mainly Sunni Muslim rebels against President Bashar Assad, whose Alawite faith is an offshoot of Shiite Islam.

"In a disturbing and dangerous trend, mass killings allegedly perpetrated by Popular Committees have at times taken on sectarian overtones," the UN commission of inquiry on Syria, led by Brazilian Paulo Pinheiro, said in its latest report to the UN Human Rights Council in Geneva.

The investigators, who cited accounts from witnesses and victims, also said people were being harassed or arrested by the committees because they came from regions perceived as being supportive of the uprising.

Both sides in the conflict, which is mired in a "destructive stalemate", have committed violations against civilians, the investigators said. The bodies of those killed in massacres have been burned or dumped in rivers, they said.

Rebel forces regularly execute captured Syrian soldiers and militiamen, and have established detention centers in Homs and Aleppo, the report said.

Syria's ambassador Faysal Khabbaz Hamoui took the floor at the Council to dismiss the report as based on "partial information from untrustworthy sources", and he accused Qatar and Turkey of "supporting terrorism" in his country.