Recent reported mass executions by Syrian militant groups, especially by an Al-Qaeda-linked faction, "may amount to war crimes," UN High Commissioner for Human Rights Navi Pillay said Thursday.

Recent reported mass executions by Syrian militant groups, especially by an Al-Qaeda-linked faction, "may amount to war crimes," UN High Commissioner for Human Rights Navi Pillay said Thursday.

"While exact numbers are difficult to verify, reliable eyewitness testimony that we have gathered suggests that many civilians and fighters in the custody of extremist armed opposition groups have been executed since the beginning of this year," she said in a statement.

"Executions and unlawful killings are in violation of international human rights and humanitarian law and may amount to war crimes," the statement said.

The Islamic State in Iraq and the Levant (ISIL) have clashed with the other militant groups as their military confrontation has left hundreds of deaths.

ISIL has seized control of Raqa, a city in Syria's north.

The UN High Commissioner for Human Rights said reports in the past two weeks have highlighted mass executions of civilians and fighters in Raqa, Aleppo and Idlib by hardline guerrillas, in particular by ISIL.
"Deeply disturbing reports" from Raqa suggested "mass executions" by ISIL, Pillay said.

Some of the killings in Syria appeared to have been carried out when 'rebel' fighters abandoning bases slaughtered people in their custody, in violation of the Geneva Conventions, she said.

"These reports are particularly alarming, given the large numbers of people, including civilians, that armed opposition groups in Syria are believed to be holding in custody. The taking of hostages is prohibited under international humanitarian law and may also constitute a war crime," she said.

"Everyone involved in serious crimes must be held accountable," Pillay said.