A deal to evacuate rebel fighters and civilians from eastern Aleppo has stalled, with heavy shelling reported.
Syrian activists say air strikes have resumed over rebel-held territory, where at least 50,000 civilians remain.
The UN said raids by the Syrian government and its allies on an area "packed with civilians" most likely violates international law.
The renewed violence came despite a ceasefire put in place less than a day earlier.
Zeid Raad al-Hussein, the UN High Commissioner for Human Rights, said the Syrian government had "a clear responsibility to ensure its people are safe".
In a statement, he said: "The way this deal was dangled in front of this battered and beleaguered population - causing them to hope they might indeed live to see another day - and then snatched away just half a day later is also outrageously cruel."
Meanwhile, the BBC reports that Western forces are using satellites and unmanned aircraft to gather evidence of possible war crimes in Aleppo and elsewhere in Syria.
Eastern Aleppo has been held by the rebels since 2012. But they have been squeezed into ever-smaller areas in recent months by a major government offensive, backed by Russian air power.
In recent weeks, government troops have made sweeping gains. Russia's military said on Wednesday that rebels were confined to only 2.5 sq km of the city.
Under the evacuation deal brokered by Russia and Turkey, civilians and rebels from eastern Aleppo were to be allowed to go to rebel-held areas in northern Syria.
Fighting ceased late on Tuesday, but evacuations scheduled for 5am on Wednesday did not go ahead, and buses left empty.
Fresh shelling was reported several hours later.
The breakdown of the deal is being attributed to demands from the government side.
It is said to be seeking the evacuation of injured fighters and civilians from nearby towns encircled by opposition forces.
"The clashes are violent and bombardment is very heavy... it seems as though everything [the ceasefire] is finished," Rami Abdulrahman, director of the British-based Syrian Observatory for Human Rights (SOHR) monitoring group, said.
The SOHR also said aircraft had resumed bombing in rebel-held areas.
Russia claims rebels refused ceasefire
Russia's Foreign Minister Sergei Lavrov said Syria's government had been happy to allow safe passage to rebel fighters, but that the rebels had refused a ceasefire.
Turkish President Recep Tayyip Erdogan accused Syrian forces of breaking the deal and called the situation in Aleppo "very fragile". He said civilians had to be allowed to leave.
Besieged residents have faced weeks of bombardment and chronic food and fuel shortages. Medical facilities in the city have largely been reduced to rubble.
Some information continues to emerge from besieged areas. Ibrahim Abu-Laith, a spokesman for the White Helmets volunteer rescue group, said more than 40 people were injured in eastern Aleppo
"Bombing is ongoing, no one can move," one activist, Mohammad al-Khatib told AFP. "The wounded and dead are lying in the street. No one dares to try and retrieve the bodies".
Residents say they fear reprisals from government forces or being forcibly conscripted. There have also been reports of atrocities.
On Tuesday the UN said it had received reliable evidence of summary executions taking place, saying that in four areas 82 civilians were killed by pro-government forces. It said that many more may have died.
How many people are in eastern Aleppo?
It is not clear exactly how many people remain in the besieged areas. UN envoy Staffan de Mistura put the figure at about 50,000.
He said there were approximately 1500 rebel fighters, about 30 percent of whom were from the jihadist group formerly known as the al-Nusra Front.
Other local sources say there could be as many as 100,000 people, many of them arriving from areas recently taken by the government.
Syria's government and Russia said the allegations were untrue.
World Vision response manager for northern Syria Christine Latif told Morning Report most buildings in the city had been raised, there was no food, no water, no electricity and no medical assistance.
"They're absolutely terrified and in the last 12 hours we've been seeing people sending goodby messages to their family on Twitter."
It was around 4°C in Aleppo at the moment, she said, and people were burning whatever they could get their hands on to stay warm.
Ms Latif said a bottle of water in Aleppo was around $USD10 - if it was available. She said the situation was disturbing.
"When you look at what's happening to people on the ground, and they're absolutely losing hope, a lot of people are talking about the world having forgotten them and having given up on them."
- BBC / RNZ