The United Nations Commission of Inquiry (COI) for Syria documented the violations in a report covering July 21 to December 22 last year -- the regime's five-month siege of eastern Aleppo, formerly the opposition's key stronghold.
"The scale of what happened in Aleppo is unprecedented in the Syrian conflict", COI chairman Paulo Pinheiro told reporters in Geneva.
The Syrian air force and its Russian ally "conducted daily air strikes" on Aleppo, an iconic city that was once the country's economic capital but has now been reduced to rubble, the COI said.
There is conclusive evidence that Syrian aircraft dropped "toxic industrial chemicals, including chlorine", the inquiry found. However, no information indicates the Russians used chemical weapons, it added.
The bombardments saw hospitals, markets and residential buildings repeatedly hit.
In a major new finding, investigators also said there was proof that Damascus was responsible for September 19 air strikes in Aleppo province that deliberately targeted a humanitarian convoy, killing at least 10 aid workers.
Civilians had 'no option'
President Bashar al-Assad's government has categorically denied responsibility for the bombardment in Urem al-Kubra and a separate UN probe in December said it was impossible to establish blame.
But after analysing satellite images, forensic evidence and other material the COI determined that "Syrian air forces targeted (the) humanitarian aid convoy".
The evidence "strongly" suggests that the attack -- which forced relief workers to pause deliveries of relief supplies -- was "meticulously planned and ruthlessly carried out" to hinder humanitarian work, according to the inquiry.
The Syria COI, set up in 2011 to investigate the most serious crimes committed in the country's civil war, was asked by the UN rights council in October to specifically probe the battle for Aleppo.
The latest findings were released in Geneva, where the UN was also hosting regime and opposition delegations for faltering peace talks.
Pinheiro voiced frustration that Assad's government did not cooperate with the probe, even after he told Damascus "it was in their interest to have their narrative" included in the findings.
Investigators described Aleppo as a scene of "unrelenting violence" in which civilians in the rebel-held east and government-controlled west fell "victim to war crimes committed by all parties".
The disparate rebel factions in Aleppo, including former Al-Qaeda-affiliate Fatah al-Sham Front, shelled civilians in western Aleppo and indiscriminately fired with no clear military target, the COI said.
Reprisals
As the opposition resistance was crumbling and civilians tried to escape, "some armed groups violently prevented them and used them as human shields", the inquiry found.
The evacuation deal agreed between warring parties that marked regime's victory allowed for civilians to move into western Aleppo or be transported to Idlib, an opposition stronghold in Syria's northwest.
Those evacuations, which were observed by UN staff and the International Committee of the Red Cross, left civilians with "no option to remain", the COI said.
"Such agreements amount to the war crime of forced displacement of the civilian population", UN investigators concluded, stressing that the deal in Aleppo was made "for strategic reasons – and not for the security of civilians."
The Aleppo fight was among the most horrifying episodes in a six-year war that has claimed more than 310,000 lives. Medical charity Doctors Without Borders described the city's east as a "kill box."
The battle began to decisively turn in the regime's favour in late November, setting off a series of reprisal killings perpetrated by both Syrian soldiers and allied fighters.
"These included cases of Syrian soldiers killing their own relatives" who backed the opposition, and pro-government forces executing the wife and daughter of a rebel commander as they tried to enter western Aleppo, the report said.
Assad's forces have reportedly been backed by Lebanese, Iranian and Iraqi forces among others, but the COI did not decisively link individual militias to specific atrocities.
