A ceasefire agreement to evacuate tens of thousands of fighters and civilians from the remaining rebel-held pockets of eastern Aleppo has been suspended, according to a Syrian official.
Speaking to Reuters news agency on Friday, the Syrian government official overseeing the operation said it was suspended due to "obstructions".
At the time of the deal's suspension, the government-run SANA reported that more than 8,000 residents of eastern Aleppo, among them fighters, had been evacuated.
State TV reported that rebel groups were attempting to smuggle prisoners and heavy weapons out with them, violating the evacuation deal.
The opposition media outlet Orient TV, however, accused Shia militias loyal to the Syrian government of opening fire on the convoy. Speaking to Reuters, a rebel commander confirmed the claim.
A Reuters witness heard at least four blasts at a location where buses had been departing.
As part of an agreement between Turkey and Russia, tens of thousands of rebels and civilians were supposed to be evacuated from eastern Aleppo to rebel-held Idlib, allowing the Syrian government to take full control of the city after years of fighting.
The operation began on Thursday and was expected to take several days.
The agreement also allowed for the evacuation of thousands of civilians from Kefraya and Fua, two Shia-majority towns besieged by rebel forces.
Rebel forces allegedly shelled the towns on Friday morning around the same time the evacuation from eastern Aleppo was halted.
Al Jazeera's Imtiaz Tyab, reporting from Gaziantep on the Turkish side of the Turkey-Syria border, said there were conflicting reports of who started the fighting.
"According to several sources, an Iranian-backed militia blocked a road that the evacuees would be using to reach the Aleppo countryside. It then started firing," he said.
"According to the reports, they were protesting against this evacuation deal, which would see the villages of Fua and Kefraya evacuated in a similar way east Aleppo was."
A Turkish official told Turkish news agency Anadolou that Syrian government forces had arrested at least 800 people before the suspension of the agreement.
Zouhir Al Shimale, a journalist in eastern Aleppo, reported that the evacuation had been moving at a snail's pace.
"People were outside in the street overnight," he told Al Jazeera on Friday morning, adding that the temperature had dipped below zero degrees Celsius in the city. "Thousands are still waiting out here."
US Secretary of State John Kerry has warned that action is needed to prevent a potential massacre.
The withdrawal began a month to the day after Syrian government forces launched a major offensive to retake all of Aleppo, and will hand the Syrian President Bashar al-Assad his biggest victory in more than five years of civil war.
The city, Syria's second largest city and once a key cultural and economic hub, has been divided between government forces and rebels since 2012.
In a video message to Syrians, Assad said the "liberation" of Aleppo was "history in the making".