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.
"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 on Monday.
"Some appear to have been trained and armed by the government," they said.
The independent investigators, who cited accounts from witnesses and victims, said people were being harassed or arrested by government-allied militia because they came from regions perceived as being supportive of the revolt.
Popular Committees have been documented as operating across Syria, "where at times they are alleged to be participating in house-to-house searches, identity checks, mass arrests, looting and acting as informants," they said in a 10-page report.
The conflict is mired in a "destructive stalemate" amid heavy shelling and air raids by government forces, they said.
Both sides have committed violations against civilians, the U.N. investigators said. They were pursuing probes into about 20 cases of massacres, including three in Homs at the start of the year, despite their lack of access to the country.
The bodies of some of those killed in massacres have been burned or dumped in rivers, they said.
"TIDAL WAVE" OF DISPLACEMENT
"Indiscriminate and widespread shelling, the regular bombardment of cities, mass killing and the deliberate firing on civilian targets have come to characterize the daily lives of civilians in Syria," Pinheiro said.
Hospitals have been targeted and medical staff arrested, he said, denouncing the use of medical care "as a tactic of war".
Pinheiro called for a political solution to the crisis which he said had set off a "tidal wave" of displacement.
More than 1 million Syrian refugees have fled abroad and 2.5 million are uprooted within the country, while more than 70,000 people have been killed, according to the United Nations.
Rebel forces often execute captured Syrian soldiers and militiamen, and have established detention centers in Homs and Aleppo, the report said.
Rebels have also taken up positions in or near densely populated areas, in violation of international law, it said.
Syrian warplanes bombed the shattered Baba Amr district in the central city of Homs on Monday, a day after rebels made a surprise push into their former bastion, which had been in army hands for a year.
Syria's ambassador Faysal Khabbaz dismissed the U.N. report as based on "partial information from untrustworthy sources" and accused Qatar and Turkey of "supporting terrorism" in Syria.
"There is a conspiracy against Syria. Qatar has financed and armed tens of thousands of mercenaries from 30 countries. Turkey has provided the military bases and sent them into Syria on their jihad," Khabbaz Hamoui said.
Russia called the report "unbalanced" and said there was much evidence to show that the armed opposition was using sexual violence as a weapon and training child soldiers in camps.
The European Union and United States denounced continuing crimes and said that those responsible must be held accountable.
"If concerns about war crimes and crimes against humanity are not adequately addressed on a national level, the International Criminal Court should deal with the situation," said EU ambassador Maria Angela Zappia.
U.S. ambassador Eileen Chamberlain Donahoe condemned the "regime's brutality" and said: "We are also deeply concerned by reports of abuses by opposition-affiliated forces and the presence of foreign forces and violent extremists who are attempting to hijack the legitimate struggle of the Syrian people."