BEIRUT: A convoy of 60 buses carrying people desperate to leave the last rebel-held enclave of Aleppo was held up in freezing temperatures on Wednesday when an evacuation deal hit a last-minute hitch.

The eventual departure of the thousands left in the insurgent zone will hand full control of the city to Syrian President Bashar al-Assad, the biggest prize of the nearly six-year-old civil war.

The Syrian Observatory for Human Rights, a British-based monitoring group, said the bus convoy entered the last besieged pocket of the city on Tuesday in harsh weather conditions and some were loaded with people, but they had not moved since.

Eight buses en route to Aleppo from the pro-government villages of Foua and Kefraya had also been held up since Tuesday, the Observatory said. Government forces had insisted the two villages, besieged by rebels in Idlib, must be included in the deal to bring people out of east Aleppo.

State television, broadcasting live from near the Aleppo departure zone, said the rebels had presented new conditions with the aim of obstructing the evacuation deal. Rebels said the government was to blame for hindering the evacuations.

So far, about 25,000 people have been evacuated from Aleppo, according to the International Committee of the Red Cross, which is helping with the process. A UN official said 750 people had so far been evacuated from Foua and Kefraya.

Behind those leaving Aleppo's rebel zone was a wasteland of flattened buildings, concrete rubble and bullet-pocked walls, where tens of thousands lived until recent days under intense bombardment even after medical and rescue services had collapsed.

Rebel-held parts of the once-flourishing economic centre with its renowned ancient sites has been pulverised during the war which has killed more than 300,000 people, created the world's worst refugee crisis and allowed for the rise of Islamic State.

But in the much larger western part of the city, held throughout the war by the government, there were big street parties on Tuesday night, along with the lighting of a Christmas tree, as residents celebrated the end of fighting.

The Syrian army has used loudspeakers to broadcast warnings to rebels that it was about to enter their rapidly diminishing area and told them to speed up their evacuation.

Control of Aleppo would be a major victory for Assad, and his main allies Iran and Russia, against rebels who have defied him in Syria's most populous city for four years.

UN monitors evacuation

The United Nations had said it had sent 20 more staff to east Aleppo to monitor the evacuation.

"Some have arrived yesterday (Tuesday) and more will be arriving today and in the coming days," Jens Laerke, UN spokesman in Geneva told Reuters.

Both combatants blamed the other side for the delay.

Ahmad al-Dbis, a medical aid worker heading a team evacuating patients from Aleppo, also said the east Aleppo convoys had been due to head earlier towards the rebel-held countryside.

"People are waiting in the buses, and the buses are not heated, and it's very cold," he added. "Many of the evacuees told us they had been stopped for more than 20 hours in the cold and snow."

Various problems have beset the evacuation, with estimates of how many have left and how many remain varying widely.

The Observatory said nearly 3,000 people still remain in the east Aleppo enclave, including mostly rebels and some civilians.

Anti-assad resistance

Assad's government is backed by Russian air power and Shi'ite militias including Lebanon's Hezbollah movement and Iraq's Harakat al-Nujaba. The mostly Sunni rebels include groups supported by Turkey, the United States and Gulf monarchies.

For four years, the city was split between a rebel-held eastern sector and the government-held western districts. During the summer, the army and allies forces besieged the rebel sector before using intense bombardment and ground assaults to retake it in recent months.

Russian air strikes were the most important factor in Assad's triumph. They enabled his forces to press the siege of eastern Aleppo to devastating effect and regain control of what was Syria's biggest city and economic hub before the war.

On the ground, Shi'ite militias from as far afield as Afghanistan played an important role for Assad, a member of the minority Alawite sect which is an offshoot of Shi'ite Islam.

Victory in Aleppo leaves Assad virtually unassailable by the rebels but he still faces great challenges in restoring the power of his state. While he controls the most important cities in western Syria and the coast, armed groups including Islamic State control swathes of territory elsewhere in Syria.