"There needs to be a thorough independent investigation to establish whether war crimes have been committed. And those responsible for such crimes should be brought to justice," Navi Pillay said on Friday.

She added that the killing of Syrian soldiers taken captive in the town of Khan al-Assal in the northern province of Aleppo was "deeply shocking".

Dozens of members of Syrian forces were reported to have been executed.

"Based on the analysis by my team to date, we believe armed opposition groups in one incident -- documented by a video -- executed at least 30 individuals, the majority of whom appeared to be soldiers," Pillay stated.

"Opposition forces should not think they are immune from prosecution. They must adhere to their responsibilities under international law," she noted

In late July, armed terrorist groups killed at least 123 people, including security forces and civilians, during an attack on Khan al-Assal. They mutilated their corpses, threw them into a crater outside town and incinerated the bodies of a number of them.

Western powers and their regional allies - including the Israeli regime, Turkey, Saudi Arabia and Qatar - continue to support militant groups, including al-Qaeda-linked terrorists, in Syria.

The conflict in Syria started in March 2011, when pro-reform protests turned into a massive insurgency following the intervention of Western and regional states.

The unrest, which took in terrorist groups from across Europe, the Middle East and North Africa, has transpired as one of the bloodiest conflicts in recent history.

As the foreign-backed insurgency in Syria continues without an end in sight, the US government has boosted its political and military support to Takfiri extremists.

Washington has remained indifferent about warnings by Russia and other world powers about the consequences of arming militant groups.

SHI/SHI