The Syrian government is reportedly using local militias known as Popular Committees to commit mass killings, U.N. human rights investigators said on Monday. 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. But both sides in the conflict have committed violations against civilians, the investigators said.

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

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 al-Assad, whose Alawite faith is an offshoot of Shi'ite Islam.