Rescuers at the site of a collapsed factory in Pakistan have continued to dig through the rubble, as the death toll rose to at least 21.

Rescue officials said 102 survivors had so far been pulled from the wreckage of the factory, which manufactured polythene shopping bags 12 miles south of the eastern city of Lahore, and collapsed on Wednesday night.

At least 21 bodies had also been recovered by rescue crews using heavy construction machinery and other tools to dig through the debris.

Earlier, officials estimated 150 people had been in the building when it collapsed. Rescuers had to move slowly to avoid further injuries to those still trapped.

Injured survivors said the factory's owner, who was adding a new floor to the building, had ignored advice from his contractor and pleas from his workers to stop construction after cracks in the walls following a powerful earthquake last week.

The quake of magnitude 7.5 killed more than 300 people in Pakistan and the northern parts of neighbouring Afghanistan and damaged thousands of buildings.

Muhammad Ramzan, 22, one of the survivors, said he had witnessed cracks first appearing in the structure, moments before the collapse.

"Suddenly, I saw cracks appearing in the pillars. I immediately drew the owner's attention towards the cracks. He was watching them when the roof collapsed and I saw him being crushed by a heap of concrete that led to his death," he said.

Many survivors were able to guide rescuers by calling relatives using their mobile phones.