An explosion tore through an illegal fireworks warehouse Saturday, killing at least 13 people and setting fire to several nearby shops and buses in a crowded bazaar.

Hundreds of people burrowed through smoldering debris looking for survivors after the blast flattened a three-story building in Rawalpindi, nine miles from the capital of Islamabad.Three of the dead were children. At least 57 were injured, many critically, hospital officials said.

"We're worried about another collapse," said Azim Hussein, a coordinator of the rescue operations in the congested Raja Bazaar.

Ambulance workers said it was likely the death toll would increase as many of the injured were in critical condition, and it wasn't known how many people might be buried beneath the rubble.

Hundreds dug frantically for survivors.

"We heard the explosion and everything shook," said Mohammed Sohail, a local shopkeeper. "There were clouds of dust, and everyone tried to clear the rubble."

District Magistrate Nasir Saeed said the fireworks manufacturer was operating illegally in the building and had been warned that his premise was unsafe.