A fresh UN report on Syria has shed light on war crimes committed by both sides as the civil war becomes increasingly sectarian.

The report, by the UN's commission of inquiry on Syria, covers a period from 15 May to 15 July 2013 and confirms at least eight massacres perpetrated by president Bashar al-Assad's regime and one by the rebels opposing him.

The four-member commission found evidence of attacks on the civilian population, with government forces committing murder, torture, rape and abduction. Anti-government groups have committed war crimes such as murder, execution without due process, hostage-taking and attacking protected objects.

"The intentional mass killing and identity of the perpetrator were confirmed to the commission's evidentiary standards," reads the report.

The UN commission said it carried out 2,091 interviews, most of them from the 2 million Syrian refugees pouring over the border into neighbouring countries.

The most recent massacre, which is emblematic of the sectarian dimension of the conflict, took place in Hatla, near the eastern city of Deir el-Zour.

The rebels outnumbered and killed 20 Shia fighters and took over the town. Anti-government troops then conducted home invasions, killing and executing many Shia including at least 30 civilians - among them children, women and the elderly.

A girl stands in front of a building damaged by what activists said was shelling by forces loyal to Syria's President Bashar al-Assad
A girl stands in front of a building damaged by what activists said was shelling by forces loyal to Syria\'s President Bashar al-Assad (Reuters)

"Fighters also set civilian houses and a Shia mosque on fire as they shouted sectarian slogans," reads the report. "There are reasonable grounds to believe that the anti-Government fighters who attacked Hatla unlawfully killed at least 20 civilians in violation of international law."

However the worst massacre was committed by government forces in May, according to the UN report.

Assad's troops raided the government-controlled village of Ras al-Nabe, near the coastal town of Banias, seizing and killing civilians. The total death toll was around 150-200, including children, with many people suffering deadly blows from heavy or sharp objects in the face and head.

"Testimonies of those who witnessed the aftermath described bodies lying in the streets for days before the inhabitants could safely return to collect them," reads the report.

"As in Al-Bayda, there is no indication that the armed opposition was active in the village. The operation did not occur in the context of a military confrontation."