The U.N. human rights office says that a "soaring" number of executions in northern Syria could amount to war crimes.

In a statement, U.N. High Commissioner for Human Rights Navi Pillay warned armed opposition groups that such executions violate international humanitarian law.

Pillay said Thursday that over the past two weeks her office has received reports of "a succession of mass executions of civilians and fighters who were no longer participating in hostilities in Aleppo, Idlib and Raqqa by hardline armed opposition groups in Syria, in particular by the" al-Qaida linked Islamic State of Iraq and Syria.

Pillays says exact numbers are hard to verify but that "many civilians and fighters in the custody of extremist armed opposition groups have been executed since the beginning of this year."