The UN's Commission of Inquiry for Syria (COI) said Wednesday it had received reports that opposition fighters were blocking civilians from fleeing Aleppo and using them as human shields.
In a statement, the COI said it had "allegations of opposition groups... preventing civilians from leaving as well as opposition fighters embedding themselves within the civilian population, thus heightening the risk to civilians of being killed or injured".
It specifically implicated Al-Qaeda's former affiliate Fateh al-Sham Front and the powerful Ahrar al-Sham rebel group.
The COI, an independent investigative body set up by the UN in 2011, collects evidence on human rights law violations committed during Syria's civil war.
In October, the UN human rights council mandated the COI to launch a special investigation into the Aleppo crisis.
Since then, government troops and their allies have waged a brutal offensive on Syria's second city pushing the rebel resistance to the brink of collapse.
The COI also noted the grave allegations levelled against pro-government forces in Aleppo, including "summary executions, arbitrary arrests, enforced disappearances and forced conscription".
The commission said Syrian President Bashar al-Assad's government was now in "effective control" of eastern Aleppo, a former rebel stronghold, and therefore had "primary responsibility" for ensuring that violations in the city are stopped.
And, with the deal struck late Tuesday for the evacuation of rebels from Aleppo on hold, the COI echoed a call made across the humanitarian community that civilians be allowed to leave conflict zones safely.
In a separate statement, UN rights chief Zeid Ra'ad al Hussein called the breakdown of the evacuation deal "outrageously cruel" and "inexcusable" as it offered civilians false hope that the fighting would stop.
Zeid noted there were disagreements as to why the deal fell through, but said Wednesday's "resumption of extremely heavy bombardment by the Syrian Government forces and their allies on an area packed with civilians is almost certainly a violation of international law and most likely constitutes war crimes."
