Hopes have risen that the evacuation of rebel-held parts of Aleppo will now begin soon, following statements by rebel and pro-government sources.
Rebel fighters and civilians in the Syrian city had been due to leave early on Wednesday, but a truce collapsed.
Rebel sources said a new truce had been in effect from 03:00GMT and evacuations would take place early on Thursday.
Sources from the Syrian military, the pro-government Hezbollah and Russian media said preparations were under way.
Reuters news agency quoted one Syrian official source on Thursday morning as saying that the “operation to organise the departure of gunmen from eastern Aleppo has now started”.
A media unit run by Hezbollah, the Lebanese Shia Muslim movement backing the Syrian government, said there had been “big complications” but that “intensive contacts between the responsible parties… led to re-consolidating a ceasefire to exit armed fighters from eastern districts in the next few hours”.
Soldiers from Russia – Syria’s ally – would lead the rebels out, escorting them on a corridor towards Idlib city on buses and ambulances, with surveillance drones monitoring the situation, Russian media said.
A statement from the Russian Centre for the Reconciliation of Opposing Sides in Syria, part of Russia’s ministry of defence, said Syrian authorities had guaranteed the safety of all members of the armed groups who decided to leave Aleppo.
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Ismail Alabdullah, a volunteer for the White Helmet rescue group, told the BBC that buses had now entered the area in which he was working and that he hoped the evacuation would now take place.
The UK-based Syrian Observatory for Human Rights monitoring group said that the first convoy of wounded people had now started to move from eastern to western Aleppo.
BBC Arabic’s Asaf Aboud, in Aleppo, says there was some shelling by rebels and air strikes by government forces overnight.
The new deal should 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 had been 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.
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 appeared to upset Iran as well as the Syrian government – they 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.
Only when buses are boarded, and ambulances pull away, can it be said with any certainty that this battle is drawing to a close.
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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.
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.