It said the images, obtained from commercial satellites over July 23- to August 1 period, showed more than 600 craters, probably from artillery shelling, dotting Aleppo's surrounding areas. The craters were represented with yellow dots in the images.

One snapshot, from July 31, showed craters next to what looked like a residential housing complex in the nearby town of Anadan, it said.

Amnesty said the other photos showed a giant queue of cars and trucks waiting at a gas station as supplies become depleted in the city and a truck on fire in a roadblock probably manned by civilians, as well as a general overview of the city showing hotspots of military presence and checkpoints.

Scott Edwards, who oversees Amnesty's satellite technology programme, said schools, mosques, churches and hospitals inside the city appeared to be untouched by the shelling for now but there was a considerable build-up in pro-Assad forces.

He said the satellite images showed at least 58 tanks deployed in the area as well as 45 armoured personnel carriers and other infantry vehicles and numerous artillery units.

Assad has been reinforcing troops in preparation for an assault to recapture rebel-held districts of Aleppo after repelling fighters from most of the capital Damascus.

On Wednesday, rebels abandoned some of their positions and explosions could be heard in the area as helicopters flew over frontline areas, a Reuters reporter there said.