UN human rights investigators say Syrian government forces and their allied Shabbiha militia have committed war crimes and crimes against humanity in what appears to be a state-directed policy.
The panel said on August 15 that Syrian rebels fighting against President Bashar al-Assad's regime had also committed war crimes but these "did not reach the gravity, frequency, and scale" of those carried out by security forces.
The report came as fighting raged across Syria, with reports of a government air raid killing at least 20 people in a rebel-held town in the north of the country, and clashes and bomb explosions in the capital, Damascus.
In its report, the panel of independent experts appointed by the UN Human Rights Council blamed security forces for the killing of more than 100 civilians in the village of Houla in May, nearly half of them children.
It said security forces were also responsible for other "gross violations of international human rights law" including murder, torture, arbitrary arrest, unlawful killing, and indiscriminate attacks against civilians.
Damascus did not allow the team of UN investigators into the country. The report, which covers events between February 2012 and late July, is based on interviews with hundreds of Syrian refugees in neighboring countries.
The panel recommends that the Human Rights Council forward the report to UN Secretary-General Ban Ki-moon, who could bring it to the attention of the UN Security Council.
In Moscow, Russian Foreign Minister Sergei Lavrov accused the West of reneging on a peace plan to resolve the Syria crisis and of prolonging the violence by encouraging the rebels.
"It is essential that all external players put pressure on all Syrian sides and stop urging the opposition to continue their military struggle, which will make the government agree to a six-point plan and the Geneva agreement," Lavrov said.
"By the way, the government expressed their readiness to do this if the opposition agrees to the same. But the opposition hasn't agreed just yet."
Syrian activists say more than 20,000 people have been killed in the 17-month revolt against Assad's regime in Syria.
Violence Across Syria, Blast In Damascus
Meanwhile, activists said at least 20 people were killed in a government air strike on the rebel-held northern town of Azaz on August 15.
The Britain-based Syrian Observatory for Human Rights said the air strike hit Azaz, a rebel-held northern border town near the main northern city of Aleppo.
Earlier, a huge explosion occurred close to a military building and a hotel used by UN monitors in Damascus.
State media said the blast was caused by a bomb and three people were wounded.
Syrian Deputy Foreign Minister Faisal al-Mokdad said that UN observers were unhurt in the explosion.
The rebel Free Syrian Army reportedly claimed responsibility for the attack.
Syrian activists and pro-government media also reported clashes between security forces and rebels in Damascus.
Activists said the fighting erupted when rebels attacked security force checkpoints in the western Damascus district of Mezze, near government buildings and the Iranian Embassy. Pro-government media reported several opposition fighters were killed or arrested.
OIC To Kick Out Syria
In Mecca on August 15, an emergency summit of the 57-member Organization of Islamic Cooperation is poised to suspend Syria from the group. Iran strongly opposes the move.
The symbolic gesture is expected to pile pressure on Damascus over its deadly crackdown on a 17-month uprising against President Bashar al-Assad's regime.
A draft final statement obtained by AFP said the summit "approves the suspension of Syria's membership." It is expected to be endorsed when the leaders reconvene later on August 15.
On August 14, U.S. Defense Secretary Leon Panetta accused Iran of playing a greater role in the Syrian conflict, including efforts to train a militia to fight alongside Assad's forces.
"We are seeing a growing presence by Iran and that is of deep concern to us, that that's taking place," Panetta said. "We do not think that Iran ought to be playing that role at this moment in time, that it's dangerous, that it's adding to the killing that's going on in Syria, and that it tries to bolster a regime that we think, ultimately, is going to come down. But all it's going to wind up doing, frankly, is to prolong the misery of the Syrian people."
The chairman of the U.S. Joint Chiefs of Staff, General Martin Dempsey, told reporters the militia is being used to take pressure off Syrian regime forces.
With reporting by Reuters, AP, dpa, and AFP