Both sides in last year's battle for Syria's Aleppo city committed war crimes, including a "deliberate" bombing of a humanitarian convoy by the Syrian government, according to a new United Nations investigation.

The UN Commission of Inquiry's report released on Wednesday said Syrian government and allied Russian forces "pervasively used" unguided munitions to bomb densely populated areas in rebel-held eastern Aleppo between July and its fall on December 22, amounting to the war crime of indiscriminate attacks.

These included aerial bombs, air-to-surface rockets, cluster munitions, incendiary bombs, barrel bombs, and weapons delivering toxic industrial chemicals.

But investigators could not say whether both Syrian and Russian forces had used them in Aleppo or only one had. Neither did they attribute any specific war crime investigated to Russian forces.

"Throughout the period under review, the skies over Aleppo city and its environs were jointly controlled by Syrian and Russian air forces ... (They) use predominantly the same aircraft and weapons, thus rendering attribution impossible in many cases," the report said.

Aleppo, once Syria's largest city and former commercial hub, had been divided into rebel and government parts since 2012.

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The recapture of its eastern sector in late December by government forces was the biggest blow to Syria's rebel movement since fighting started in 2011.

Syrian helicopters unleashed toxic chlorine bombs "throughout 2016" on Aleppo, a banned weapon that caused hundreds of civilian casualties there, the report said.

At least 5,000 pro-government forces also encircled eastern Aleppo in a "surrender or starve" tactic, it said.

In a major new finding, the investigators also accused the Syrian government of a "meticulously planned and ruthlessly carried out" air strike on a UN and Syrian Red Crescent convoy at Orum al-Kubra, in rural western Aleppo on September 19 which killed 14 aid workers.

President Bashar al-Assad's government has fiercely denied responsibility for the convoy's bombardment and a separate UN probe in December said it was impossible to establish blame.

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But after analysing satellite images, forensic evidence and other material, the inquiry determined that "Syrian air forces targeted (the) humanitarian aid convoy".

"By using air-delivered munitions with the knowledge that humanitarian workers were operating in the location, Syrian forces committed the war crimes of deliberately attacking humanitarian relief personnel, denial of humanitarian aid, and attacking civilians," the report said.

During the recapture of eastern Aleppo, pro-government forces arrested doctors and aid workers and committed reprisal executions, the report said.

Crimes on rebel side

According to the report, opposition groups shelled government-controlled western Aleppo, indiscriminately firing with no clear military target.

As the opposition resistance was crumbling and civilians tried to escape, some rebel armed groups prevented civilians from fleeing eastern Aleppo, using them as "human shields", and attacked the residential Kurdish district of Sheikh Maqsoud - both war crimes.

The US-led coalition did not conduct any offensive air missions over Aleppo in the second half of the year, the UN investigators said.

The findings - released as Syrian peace talks continue in the Swiss city of Geneva - was based on 291 interviews with victims and witnesses, as well as analysis of forensic evidence and satellite imagery.

Al Jazeera's Mohammed Jamjoom, reporting from Geneva, said the authors of the report stated that there was "just no accountability".

"Even though this is being documented, even though the world knows this is going on ... UN investigators said the fact of the matter is no side in this conflict feels that they are accountable when it comes to committing these atrocities," he said.

"The bigger question now is, if this is documented, what is going to happen?" he said.

"What the press was told was that the UN is preparing a dossier so if there is a tribunal that eventually happens, the evidence is ready to try to prosecute those who are accused of doing war crimes."

Russia and China blocked in 2014 a request for the International Criminal Court to open up investigations of war crimes committed during the war in Syria.

The Syrian government has repeatedly denied using chemical weapons in the war that has killed nearly 400,000 people and displaced almost half the country's population since 2011.