GENEVA — A recent series of mass executions attributed to jihadist rebels in Syria may constitute war crimes and crimes against humanity, Navi Pillay, the United Nations human rights chief, said on Thursday.

Mass executions of civilians and of fighters who were no longer participating in hostilities were reported in the northern cities of Aleppo, Idlib and Raqqa. They were carried out by armed opposition groups in Syria, in particular by the Islamic State of Iraq and Syria, Ms. Pillay said in a statement, citing what she described as reliable testimony by witnesses.

The executions appear to have coincided with a succession of fierce battles between opposition groups in northern Syria since the start of the year as moderate and Islamist factions try to drive out hard-line fighters linked to Al Qaeda.

Many bodies, often handcuffed and blindfolded, were found eight days ago in a children’s hospital in Aleppo used by the Islamic State of Iraq and Syria as a base until an attack by other opposition groups forced the militants to withdraw, Ms. Pillay said. A witness interviewed by the United Nations human rights office identified four local news media activists among the dead, she said.