The U.N. human rights office says armed opposition groups in Syria, particularly an extreme al-Qaida-linked faction, are committing a "soaring" number of killings in the country's north that could amount to war crimes.
U.N. High Commissioner for Human Rights Navi Pillay said Thursday that reports in the past two weeks have documented numerous mass executions of civilians and fighters no longer participating in hostilities in the cities of Aleppo, Idlib and Raqqa.
She said the killings were carried out by "hardline armed opposition groups in Syria," in particular the al-Qaida-linked Islamic State of Iraq and the Levant [ISIL].
Also Thursday, Russian Foreign Minister Sergei Lavrov said Iran "inevitably" will become part of attempts to end Syria's civil war, as Moscow strongly urged the West to invite Tehran to participate in next week's peace talks in Switzerland.
Lavrov met with his Iranian and Syrian counterparts, Javad Zarif and Walid al-Moallem in Moscow.
The U.S. has opposed the involvement of Iran, the strongest regional ally of Syrian President Bashar al-Assad.
Meanwhile, fighting on the ground in Syria continued.
The Britain-based Syrian Observatory for Human Rights said the two weeks of clashes between ISIL and other rebel forces in northern Syria has killed nearly 1,100 people, including 130 civilians, some of whom were executed.
Pillay said some of the killings appear to have been carried out when rebel fighters abandoning bases slaughtered people in their custody, in violation of the Geneva Conventions.
Jihadists factions such as ISIL were initially welcomed by other rebels, but allegations of brutal abuses against civilians, as well as rival opposition fighters, has sparked a backlash.
The United Nations delivered aid to two besieged areas under opposition control near the capital, Damascus, on Thursday, a day after it said Syrian authorities had promised to allow humanitarian supplies through to areas cut off by fighting.
A U.N. spokesman said a convoy assisted by the Syrian Arab Red Crescent had brought food, medicine and winter relief supplies to Al Ghuzlaniyah, close to Damascus airport.
On Tuesday, Syria's government and the main Western-backed opposition group, the Syrian National Coalition, agreed to allow humanitarian aid into some blocked-off areas.
Fighters from the Free Syrian Army's Saif al-Umayyad brigade prepare rockets to be launched towards forces loyal to Syria's President Bashar al-Assad in the eastern Damascus suburb of Ghouta, Jan. 16, 2014.
Men ride a motorbike past buildings damaged by what activists said was shelling by forces loyal to Syria's President Bashar al-Assad in Daraya, Jan. 15, 2014.
Smoke rises from buildings after what activists said was shelling from forces loyal to Syria's President Bashar al-Assad in the besieged area of Homs, Jan. 15, 2014.
Free Syrian Army fighters walk at the Tameko pharmaceutical factory after the FSA claimed to have taken control of the factory, in eastern al-Ghouta, near Damascus, Jan. 14, 2014.
A girl carries her belongings as she walks on rubble at a site hit by what activists said was an airstrike by forces loyal to Syria's President Bashar al-Assad in Arbeen, Jan. 14, 2014.
This SANA photo shows Syrian President Bashar al-Assad speaking to female preachers from mosques in Damascus and its countryside and educators of religious high schools and institutes in Damascus, Jan. 14, 2014.
Syrian refugees wait to enter Turkey on the Syrian-Turkish border in Shamm Alqrain village, Jan. 13, 2014.
Smoke rises from what activists said were explosive barrels thrown from helicopters on Daria outside Damascus, Jan. 12, 2014.
Damaged buildings are pictured in the besieged area of Homs, Jan. 12, 2014.
Residents inspect the damage caused by Islamic State in Iraq and the Levant suicide bombers at the Tawhid Brigade and Al-Fateh brigade headquarters in Aleppo, Jan. 12, 2014.