The evacuations which were expected to begin at 0300 GMT, according to the Syrian Observatory for Human Rights monitor, had yet to commence more than half an hour after the time had passed, an AFP correspondent in the city said.

About 20 green government buses were gathered at the edges of the divided Salaheddin district. Some entered briefly Tuesday night but returned empty and parked on the outskirts again, the journalist said.

Rebel officials, Russia and Turkey confirmed the agreement which, if implemented, would mark a major victory for President Bashar al-Assad over opposition forces who rose up against him in 2011.

The agreement came amid mounting global outrage over reports of atrocities, including dozens of summary executions, as forces loyal to Assad closed in on the last pocket of rebel territory in Aleppo.

Officials from several rebel groups told AFP there was a deal to allow civilians and fighters to leave Aleppo for opposition-held territory to the west of the city.

"An agreement has been reached," said Yasser al-Youssef from the political office of the key Nureddin al-Zinki rebel group, adding it had been "sponsored" by Russia and Turkey.

"The first stage will be the evacuation of civilians and wounded, within hours, and afterwards fighters will leave with their light weapons."

Washington said it had not been consulted but would welcome any arrangement that brought safety to the people of Aleppo.

"It matters a lot less to us who or how a ceasefire is arranged or reached, and much more that one is arranged and reached," State Department spokesman John Kirby said.

"Even if it is the end of the siege in Aleppo, it is not the end of the war in Syria."

As the UN Security Council met in an emergency session to discuss Aleppo, Russian envoy Vitaly Churkin confirmed there was an agreement for the rebels to leave the city.

Turkey's foreign ministry said under the agreement civilians would first evacuate, followed by a withdrawal of rebel groups.

Deputy Prime Minister Mehmet Simsek also said on Twitter that Turkey would "set up a tent city to accommodate up to 80,000 Syrian refugees fleeing Aleppo".

An AFP correspondent in west Aleppo said there had been no sounds of fighting or bombardment since around 3:00 pm (1300 GMT) Tuesday and the Observatory also said it was quiet.

The United Nations and aid agencies had been pleading for an agreement to allow for the evacuation of thousands of civilians trapped in rebel territory.

After weeks of heavy fighting, forces loyal to Assad were on the verge of taking all of the city, dealing the biggest blow to Syria's rebellion in more than five years of civil war.