Pro-Syrian government fighters opened fire on a convoy as it prepared to leave rebel-held eastern Aleppo on Thursday, wounding at least three people, a rescue service spokesman said.
Rebels are leaving the last enclave they hold in the city after major advances by President Bashar al-Assad's forces.
"(Pro-government fighters) fired at us and at ambulance vehicles and those people opening up the road," the spokesman said.
The head of the ambulance service in the district, Ahmed Sweid, told pro-opposition Orient TV that three people had been wounded in the incident.
"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," he told Orient TV.
Early reports indicated at least one person had died. The Syrian Observatory for Human Rights, which uses a network of sources across Syria, said there were no deaths, but some of the wounded were in serious conditions.
The civil defence rescue service said on Twitter that five were wounded.
An official with an Aleppo rebel group said the first convoy had reached the Ramousah junction on the way out when they came under fire. Rebel officials said they did not complete the crossing.
Snipers fire on people
In a video interview posted to journalists, a man who said he was a civil defence worker said snipers had fired on people as they tried to open the road for the ambulances to pass a government checkpoint out of the rebel-held sector.
"The ambulances were on the way to the crossing which was specified to us to for evacuating people and the regime forces started to shoot at us...Even the men who tried to open the road with their trucks, they fired at them," the man in the video said.
A Reuters witness in nearby government-held territory heard a burst of gunfire that lasted several minutes.
According to earlier reports, at least 20 empty buses and five ambulances were waiting to pick up evacuees.
Russia, Syrian military sources and rebel officials confirmed that a new agreement had been reached after a first evacuation plan collapsed on Wednesday amid fresh fighting.
Syrian state television reported that some 4,000 rebels and their families were to be evacuated.
The agreement, brokered by Syrian regime ally Moscow and opposition supporter Ankara, will mark the end of years of fighting for control of the city and a major victory for President Bashar al-Assad.
The defence ministry in Moscow said its ceasefire monitoring centre, in cooperation with Syrian authorities, "was preparing the evacuation of the remaining rebels and members of their families from the eastern neighbourhoods of Aleppo."
It said the rebels would be evacuated toward the northwestern Syrian city of Idlib, a major opposition stronghold.
The Russian military said it was monitoring the operation with surveillance cameras and drones.
A senior Syrian military source confirmed to AFP that "preparations are happening now" for the evacuation.
Al-Farook Abu Bakr, the chief negotiator for the rebels, told AFP that the first convoy on Thursday would be only for wounded people, their carers and other civilians.
"The evacuations will be from Ramoussa" on the southern outskirts of Aleppo, the official from hardline Islamist rebel group Ahrar al-Sham said.
'Starting to board buses' 
He said an evacuation of rebel fighters would take place after the first or second convoy.
"The first batch of wounded civilians, their relatives, and some other civilian families is being prepared," said Ahmad al-Dbis, who heads a unit of doctors and other volunteers that are coordinating the evacuation of wounded people.
"The gathering point for civilians and wounded in Aleppo city is in Al-Amiriyah, and people are starting to board the buses now," Dbis said, speaking to AFP from a rebel-held area in the west of Aleppo province.
He said about 200 people were expected to be evacuated on three buses and head to a handover point in rebel-held territory.
He said that regime forces had fired on an ambulance bringing in the injured, killing one person and wounding two others.
A similar evacuation expected to take place on Wednesday morning fell apart, with artillery exchanges and resumed air strikes rocking the city until the early hours of Thursday.
Cold and hungry civilians had gathered for the evacuation but were instead sent running through the streets searching for shelter as the fighting resumed.
Russia accused the rebels of having violated the ceasefire while Turkey accused Assad's regime and its supporters of blocking the evacuation.
The new agreement on Thursday was announced a month to the day after pro-government forces launched a major new offensive to retake all of Aleppo, large parts of which had been in rebel hands since 2012.
Backed by Iranian-trained forces and fighters from Lebanon's Shia Hezbollah movement, the advance made rapid gains, seizing more than 90% of rebel territory within a few weeks.
More than 465 civilians, including 62 children, have died in east Aleppo during the assault, according to the Syrian Observatory for Human Rights.
Another 142 civilians, among them 42 children, have been killed by rebel rocket fire on government-held zones in the same period, the Britain-based monitoring group said.
The United Nations and Western countries this week condemned alleged atrocities being carried out by pro-government fighters during the advance, including reported summary executions of men, women and children.
A UN panel said on Wednesday that it had also received reports that rebel fighters were blocking civilians from leaving and using them as human shields.
More than 310,000 people have been killed since the Syrian conflict began, and over half the population has been displaced, with millions becoming refugees.