Buses evacuate thousands of exhausted Aleppo residents in ceasefire deal

(Adds Syrian White Helmets letter to U.N. investigators, paragraphs 16-18)

* Medical convoy attacked earlier

* Operation to evacuate fighters, residents under way

* Recapture of city would give Assad biggest victory of war

* Wider conflict far from over

By Laila Bassam, Suleiman Al-Khalidi and Tom Perry

ALEPPO, Syria/BEIRUT, Dec 15 (Reuters) - Thousands of people were evacuated on Thursday from the last rebel bastion in Aleppo, the first to leave under a ceasefire deal that would end years of fighting for the city and mark a major victory for Syrian President Bashar al-Assad.

A first convoy of ambulances and buses with nearly 1,000 people aboard drove out of the devastated rebel-held area of Aleppo, which was besieged and bombarded for months by Syrian government forces, a Reuters reporter on the scene said.

Syrian state television reported later that two further convoys of 15 buses each had also left east Aleppo. The second had reached the rebel-held area of al-Rashideen, an insurgent said.

The International Committee of the Red Cross said late on Thursday that some 3,000 civilians and more than 40 wounded people, including children, had already been evacuated.

ICRC official Robert Mardini told Reuters there were no clear plans yet for how to ship out rebel fighters, who will be allowed under the ceasefire to leave for other areas outside government control.

Women cried out in celebration as the first buses passed through a government-held area, and some waved the Syrian flag. Assad said in a video statement the taking of Aleppo - his biggest prize in more than five years of civil war - was a historic moment.

An elderly woman, who had gathered with others in a government area to watch the convoy removing the rebels, raised her hands to the sky, saying: "God save us from this crisis, and from the (militants). They brought us only destruction."

Wissam Zarqa, an English teacher in the rebel zone, said most people were happy to be leaving safely. But he said: "Some of them are angry they are leaving their city. I saw some of them crying. This is almost my feeling in a way."

Earlier, ambulances trying to evacuate people came under fire from fighters loyal to the Syrian government, who injured three people, a rescue service spokesman said.

"Thousands of people are in need of evacuation, but the first and most urgent thing is wounded, sick and children, including orphans," said Jan Egeland, the U.N. humanitarian adviser for Syria.

Staffan de Mistura, the U.N. special envoy for Syria, said about 50,000 people remained in rebel-held Aleppo, of whom about 10,000 would be evacuated to nearby Idlib province and the rest would move to government-held city districts.