Civilians in Syria's second city of Aleppo are suffering unthinkable atrocities, Amnesty International says.
A new report alleges that government forces and many rebel groups are committing war crimes on a daily basis.
President Assad's government has reportedly stepped up its bombardment of Aleppo in recent weeks in response to a rebel offensive. Local journalist Zaina Erhaim told the BBC that she heard the sound of children's screams and saw rescuers struggling to pull victims from the rubble.
Amnesty's report says that from January 2014 to March 2015, government aircraft launched continual attacks using barrel bombs - oil barrels, fuel tanks or gas cylinders packed with explosives, fuel, and metal fragments - on rebel-held areas of Aleppo.
Mr Assad categorically denied that barrel bombs had ever been used by his forces. Their targets included at least 14 public markets, 12 transportation hubs, 23 mosques, 17 hospitals and medical centres, and three schools.
The vast majority of fatalities from the eight attacks Amnesty investigated were civilians. According to the Violations Documentation Center, an activist-run monitoring group, barrel bombings killed at least 3,124 civilians and 35 rebel fighters in Aleppo province over the same period.
The city has been divided between government-held and rebel-held areas since fighting erupted there in 2012. Amnesty said evidence suggested the air campaign in Aleppo had "deliberately targeted civilians and civilian objects", and noted that it was a war crime to intentionally make those not directly participating in hostilities the target of attacks.
"Such a systematic attack on the civilian population, when carried out as part of government policy as appears to have been the case in Aleppo, would also constitute a crime against humanity," it added.
Armed opposition groups in Aleppo were also accused of committing war crimes by using imprecise weapons such as mortars and improvised rockets fitted with gas canisters called "hell cannons" in attacks that killed at least 600 civilians in 2014.
The report also documented widespread torture, arbitrary detention and abduction of civilians by both government security personnel and rebels in Aleppo.
Amnesty said the widespread atrocities had made life for civilians in Aleppo "increasingly unbearable", with many forced to eke out an existence underground. A resident described Aleppo as a "circle of hell".
"The streets are filled with blood. The people who have been killed are not the people who were fighting," he said.
"More than a year ago the UN passed a resolution calling for an end to human rights abuses, and specifically barrel bomb attacks, promising there would be consequences if the government failed to comply," said Philip Luther, director of Amnesty's Middle East and North Africa programme.
"Today, the international community has turned its back on Aleppo's civilians in a cold-hearted display of indifference to an escalating human tragedy."
"I saw children without heads, body parts everywhere. It was how I imagine hell to be," a local factory worker said describing the aftermath of an attack on al-Fardous district in 2014.