A deal to evacuate the last rebel-held part of eastern Aleppo is back on, opposition fighters say, a day after a previous agreement fell through.
Rebel fighters and civilians in the Syrian city had been due to leave early on Wednesday, but the truce collapsed.
Rebel groups said late on Wednesday that evacuations would take place in the early hours of Thursday.
But there has been no confirmation so far from the Syrian government or its major ally Russia.
And a media unit run by the Lebanese Shia Muslim movement Hezbollah, a backer of the Syrian government, said negotiations were undergoing "big complications" and had not yet concluded.
The rebels said the new ceasefire would come into effect late on Wednesday, with evacuations to follow hours later.
The new deal would allow the simultaneous evacuation of two villages being besieged by rebels in north-western Syria.
Syria's government and its ally Iran had insisted the evacuation from eastern Aleppo could happen only when those villages were evacuated.
On Wednesday morning, buses and ambulances were brought to evacuate rebel fighters and their families - only to be turned away shortly afterwards.
Hours after the first agreement - brokered mainly by Russia and Turkey - collapsed, air strikes 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 violated international law.
"While the reasons for the breakdown in the ceasefire are disputed, the resumption of extremely heavy bombardment by the Syrian government forces and their allies on an area packed with civilians is almost certainly a violation of international law and most likely constitutes war crimes," Zeid Raad al-Hussein, the UN High Commissioner for Human Rights, said.
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.
Most politically sensitive deal - BBC's Lyse Doucet in Beirut
Many of Syria's prolonged battles and punishing sieges have ended with a negotiated pullout of rebel fighters.
The day of departure is often marked by delays and new demands. Aleppo is no different.
But this is the most politically sensitive deal of all.
The first deal, largely brokered by Russia and Turkey, appears to have upset Iran, as well as the Syrian government, who felt they did not have enough of a say.
Both insisted, as they have done for aid convoys and evacuations elsewhere, that there must be a simultaneous mission for injured fighters and civilians in the Shia villages of Foah and Kefraya which are besieged by rebel forces.
There have been arguments over other details, too.
No sooner was a second deal announced, denials by players on one side or the other began to emerge.
Only when buses are boarded, and ambulances pull away, can it be said with any certainty that this battle is drawing to a close.
Besieged residents have faced weeks of bombardment and chronic food and fuel shortages.
Medical facilities in the city have largely been reduced to rubble, as rebels have been squeezed into ever-smaller areas by a major government offensive, backed by Russian air power.
"The wounded and dead are lying in the street," one activist, Mohammad al-Khatib, told AFP. "No one dares to try and retrieve the bodies."
It is not clear 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 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.
Meanwhile, demonstrations in solidarity with the people of Aleppo have taken place in cities across the world, including Hamburg in Germany, Sarajevo in Bosnia and Rabat in Morocco.
The lights of the Eiffel Tower were also dimmed. Paris mayor Anne Hidalgo said she hoped the gesture would highlight the need for "urgent action" to help the people of Aleppo.
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