BEIJING/DAMASCUS, Dec. 16 (Xinhua) -- While the rebels and civilians besieged in the northern Syrian city of Aleppo were evacuating in accordance with a truce deal, the United Nations would join other international organizations in monitoring the situation.

It's estimated that 15,000 people, including 4,000 rebels, were set to be evacuated from the city, where the Syrian army has already recaptured 99 percent of the rebel-held eastern part through intensive offensive, the official SANA news agency reported.

The first batch of rebels and their families reached Rashidien in the western countryside of Aleppo. The second round of evacuees also withdrew aboard buses and ambulances.

According to earlier state TV reports, the evacuation from Aleppo was supervised solely by the International Committee of the Red Cross (ICRC), without the presence of UN staff.

However, UN spokesman Stephane Dujarric said Friday that the world body has been invited to monitor and assist in the evacuation.

"It is a three-pronged evacuation: a medical evacuation of wounded and sick, as well as an evacuation of vulnerable civilians and of fighters," the spokesman said.

Jan Egeland, the UN special adviser on Syria, clarified that this is not an agreement mediated by the United Nations; rather, it is an agreement that has been made in direct talks between the parties to this war, which the United Nations was only invited this morning to monitor.

UN monitors will accompany those who are being evacuated, not only from east Aleppo but all the way to the northwestern province of Idlib, and they also stand ready to care for them all the way into Turkey, if they chose that to be their final destination, Dujarric said.

The ICRC, the World Health Organization and the Syrian Arab Red Crescent were on the ground to provide assistance to those fleeing Aleppo.

With the anticipated rebel retreat from Aleppo, the Syrian government would take complete control of the strategically located city.

Syrian President Bashar al-Assad on Thursday hailed the recapture of Aleppo as epoch-making.

"Not only the Syrian, not only the regional, but also the international situation will not be the same after the liberation of Aleppo," Assad was quoted as saying by SANA.

Once Syria's commercial hub, Aleppo has been divided roughly in two since 2012, with the western parts under government control and eastern districts held by rebels.

The situation in Aleppo has worsened rapidly after a truce accord collapsed in September, and months of heavy fighting has driven the city dwellers to the brink of humanitarian crisis.

Tens of thousands of people have fled eastern Aleppo over the past couple of weeks, with local media reports placing the number of evacuees at 85,000.