More than 450 Syrian rebel fighters and civilians have begun evacuating from three besieged towns under a rare United Nations-brokered deal between the Syrian regime and rebel groups.

The evacuation is part of a U.N.-backed six-month truce deal reached in September which provides a ceasefire to allow in humanitarian aid and will see thousands of Shiite and Sunni civilians and fighters evacuated from war zones.

More than 120 Sunni fighters, mostly wounded, from the town of Zabadani began boarding buses and ambulances bound for the Lebanese border where they will fly from Beirut to Turkey and then travel back to various opposition-held areas inside Syria, said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, according to ABC.

At the same time, about 335 Shia, including many civilians, started leaving Fuaa and Kafraya for the Syrian-Turkish border post of al-Hawa. They will fly to the Beirut airport and travel by road to the Syrian capital of Damascus, Rahman added.

Zabadani, the last major rebel stronghold on the Lebanese border, has been under attack from pro-government forces and Lebanese Shia militant group Hezbollah, while the towns of Fuaa and Kefraya, located in the northern province of Idlib, have been under siege from Sunni rebels, according to the BBC.

"The process is ongoing at a good pace," said Yacoub El Hillo, the U.N. resident and humanitarian coordinator in Syria, according to The Associated Press. Hillo said that humanitarian aid will be brought to the besieged areas over the next few days.

The International Committee for the Red Cross, the Lebanese Red Cross and the Syrian Arab Red Crescent are also involved in the humanitarian operation.

President Bashar al-Assad has agreed to several previous local ceasefires with rebel groups, which usually involved the rebel groups laying down their weapons in exchange for humanitarian aid to civilians living in the battleground areas.

The 5-year-long Syrian civil war has displaced nearly half of Syria's population and killed more than 250,000 people.