The first ambulances leaving eastern Aleppo as part of a planned evacuation have been attacked by pro-government fighters, according to a rebel official.
Rebel sources earlier said a new truce in the Syrian city had been in effect from 3am GMT (4pm NZT) and evacuations would take place in the coming hours.
Sources from the Syrian military, the pro-government Hezbollah and Russian media said preparations were under way.
Rebel fighters and civilians had been due to leave yesterday, but the ceasefire collapsed.
The rebel official told Reuters that the first ambulances had left but were shot at by militias backing President Bashar al-Assad on their way out.
The head of the ambulance service in the district, Ahmed Sweid, said three people were wounded, including a rescue worker.
"The convoy was shot at by regime forces and we have three injured, one of them from civil defence ... They were brought back to besieged areas," Mr Sweid told the pro-opposition Orient TV.
Orient TV also said a first group of wounded people had reached the Ramousah area on their way out of the eastern part of the city.
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.
White Helmet volunteer Ismail Alabdullah 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, said 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.
Buses and ambulances were brought yesterday 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, but possibly as many as 100,000, civilians remain.
The UN said raids by the Syrian government and its allies on an area "packed with civilians" most probably violated international law.
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.
It is not clear how many rebel fighters remain in the besieged areas. UN envoy Staffan de Mistura said there were approximately 1500, about 30 percent of whom were from the jihadist group formerly known as the al-Nusra Front.
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.
- BBC / Reuters