Extremist Islamist groups in Syria are committing a soaring number of executions that could amount to war crimes, the UN human rights office have said.

UN high commissioner for human rights Navi Pillay said that over the past two weeks her office had received reports of "a succession of mass executions of civilians and fighters who were no longer participating in hostilities in Aleppo, Idlib and Raqqa."

They were being carried out, Ms Pillay said, "by hardline armed opposition groups, in particular by the al-Qaida-linked Islamic State of Iraq and the Levant (Isil)."

She warned that such executions violated international humanitarian law and the numbers are thought to be alarmingly high.

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

Fighting between Isil and other rebel groups in northern and eastern Syria has killed hundreds of people over the past two weeks.

Ms Pilay's office reported that in the first week of January a number of people were executed in Idlib by armed opposition groups.

In Aleppo three people held by the group at its base in Makhfar al-Saleheen were found dead on January 6, handcuffed with bullet wounds in their heads.

Two days later, also in Aleppo, "numerous bodies, again mostly handcuffed and blindfolded, were found in a children's hospital" once used as a base by the group, the UN office said.

At least four local media activists were among the dead as well as captured fighters from opposition groups.

There also are "deeply disturbing reports emerging of mass executions" by the group, both when it withdrew from Raqqa and after it regained control earlier this week.

"These reports are particularly alarming, given the large numbers of people, including civilians, that armed opposition groups are believed to be holding in custody," said Ms Pillay.

"The taking of hostages is prohibited under international humanitarian law and may constitute a war crime."