In their latest report, investigators also blamed President Bashar al-Assad’s troops and shabiha allies for the massacre in central Syria’s Houla in May when 108 civilians, including 49 children, were killed.

A list of individuals and units believed to have perpetuated the atrocities will be submitted to the UN High Commissioner for Human Rights, they added.

“The commission found reasonable grounds to believe that government forces and the shabiha had committed the crimes against humanity of murder and of torture, war crimes and gross violations of international human rights law and international humanitarian law,” the report said.

These “violations were committed pursuant to state policy,” added investigators.

“Large-scale operations in different govenorates, their similar modus operandi, their complexity and integrated military-security apparatus indicate the involvement at the highest levels of the armed and security forces and the government,” they said.

Shabiha militia loyal to the Assad’s regime also “acted in concert with government forces” and were responsible for “many of the crimes” detailed in the 102-page report covering mid-February to July 2012.

A particular incident singled out was the Houla massacre, in which “the commission concluded that the government was responsible for the deaths of civilians as a result of shelling the Houla area and, particularly, Taldou village.”

Rebel fighters were however not spared in the probe, which found that they were guilty of war crimes, including murder, extrajudicial execution and torture.

Nevertheless, “the violations and abuses committed by anti-government armed groups did not reach the gravity, frequency and scale of those committed by government forces and the shabiha,” investigators said in the report, which is to be presented to the UN Human Rights Council on September 17.

Overall there has been a significant deterioration of human rights in Syria since the commission’s last update to the Human Rights Council in late June, the report said, with “continuous combat, involving more brutal tactics and new military capabilities on both sides”.

This development meant that the conflict “met the legal threshold for a non-international armed conflict.”

In all at least 63 people were killed on Wednesday in Syria, where more than 23,000 people have died in violence since an uprising against the regime erupted in March 2011, said the Syrian Observatory for Human Rights.