A deal to evacuate rebel fighters and civilians from eastern Aleppo has stalled, with heavy shelling reported in the Syrian city.
A ceasefire was declared in Aleppo on Tuesday and buses brought in to ferry people out of the devastated enclave.
But fighting resumed on Wednesday. Syrian activists also say air strikes over rebel-held territory have resumed.
The breakdown of the deal, brokered by Russia and Turkey, 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 latest on the ground
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 days government troops have made sweeping gains. Russia's military said on Wednesday that rebels were confined to only 2.5sq km of the city.
Late on Tuesday, Russian UN Ambassador Vitaly Churkin told an emergency session of the UN Security Council that "military actions in eastern Aleppo are over".
Under the evacuation deal, civilians and rebels from eastern Aleppo were to be allowed to go to rebel-held areas in northern Syria.
Evacuations had been due to start at 05:00 (03:00 GMT), but did not go ahead. Fresh shelling was reported several hours later.
"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 - Syria's ally - said the Syrian army resumed firing after the rebels broke the truce. Syrian TV reported rebel rocket fire on government-held parts of the city and said at least six people had been killed.
Turkish President Recep Tayyip Erdogan accused Syrian forces of breaking the ceasefire deal and called the situation in Aleppo "very fragile". He said civilians had to be allowed to leave.
The residents still under siege
Residents of besieged eastern Aleppo have faced weeks of bombardment and chronic food and fuel shortages. Medical facilities in the city have largely been reduced to rubble.
AFP news agency reported that before the fighting had resumed, crowds of civilians, holding belongings, had gathered in the streets to await evacuation.
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
- One resident, Zohair, told the BBC there was total chaos. "We don't know how many casualties there are and if there is anyone to help them"
- "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.
Syria's government and Russia said the allegations were untrue.
Meanwhile, the BBC has learned that Western forces are using satellites and unmanned aircraft to gather evidence of possible war crimes in Aleppo and elsewhere in Syria.
How many people are in eastern Aleppo?
It is not clear exactly how many people remain in besieged areas. UN envoy Staffan de Mistura put the figure at about 50,000.
He said there were approximately 1,500 rebel fighters, about 30% 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.
High stakes - by Sebastian Usher, BBC Middle East analyst
The deal to evacuate rebel fighters and civilians out of eastern Aleppo is only the latest and most complex of such arrangements. It is also the one with the highest stakes.
Rebel-held towns in a ring around Damascus have one by one ceded in similar ways.
The choreography often follows the same pattern, with buses waiting hours or days at evacuation points as final guarantees on both sides are hammered out. Safe passage to the rebel-held province of Idlib has also been part of such deals.
More rarely, the agreements have involved reciprocal evacuations from government areas encircled by rebels. That now seems to have been made part of this deal, complicating it further.
The violence has resumed, but that does not mean that the deal is off. The rebels have reached a point of no return. Many more lives may be lost in the meantime, but it is all but certain that the fighters will leave Aleppo.