The UN Commission of Inquiry's report covers the July-December period and is based on 291 interviews with victims and witnesses, as well as analysis of forensic evidence and satellite imagery.
Addressing a press conference in Geneva on Wednesday, the UN Human Rights Council released its report into events which took place in Aleppo, Syria, last year.
It said Syrian regime aircraft "deliberately" bombed and strafed a humanitarian convoy, killing 14 aid workers, and regime helicopters dropped chlorine bombs on east Aleppo, while opposition fighters used civilians as human shields.
Even the deal struck between warring rivals to evacuate civilians following the regime's five-month siege of Aleppo, amounted "to the war crime of forced displacement of the civilian population," the UN probe said.
The report also says Syrian and Russian forces conducted "daily air strikes" on rebel-held eastern Aleppo between July and its fall on December 22, killing hundreds and destroying hospitals.
Cluster munitions were "pervasively used" and air-dropped into densely-populated areas, amounting to the war crime of indiscriminate attacks.
But it could not say whether both Syrian and Russian forces had used them in Aleppo or only one had. Investigators also did not attribute any specific war crime investigated to Russian forces.
TRT World's Sara Firth has more from Geneva.
"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.
The UN included the following findings in its report:
Toxic Chlorine Bombs
Syrian regime helicopters dropped toxic chlorine bombs "throughout 2016" on Aleppo, a banned weapon that caused hundreds of civilian casualties.
At least 5,000 pro-regime forces also encircled eastern Aleppo in a "surrender or starve" tactic.
Syrian and Russian warplanes dropped unguided munitions, known as indiscriminate "dumb bombs" rather than smart bombs that have electronic sensors to find their targets.
These included aerial bombs, air-to-surface rockets, cluster munitions, incendiary bombs, barrel bombs, and weapons delivering toxic industrial chemicals.
Attack On UN And Syrian Red Crescent Convoy
Investigators accused the Syrian regime 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 that killed 14 aid workers.
"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.
Three Stages Of Attack By The Syrian Regime
Aleppo survivors "consistently described" three stages of attack.
"First, helicopters dropped barrel bombs, which struck the warehouse and a family home nearby ... Subsequently, planes, described by several witnesses as Sukhoi jets, carried out attacks, killing several aid workers. Lastly, the aircraft fired machine guns at survivors."
During the recapture of eastern Aleppo, pro-regime forces arrested doctors and aid workers and committed reprisal executions.
Opposition War Crimes In Aleppo
Opposition groups shelled regime-controlled western Aleppo, killing and injuring dozens, the report said. They prevented civilians from fleeing eastern Aleppo, using them as "human shields," and attacked the Kurdish residential district of Sheikh Maqsoud, both war crimes.