Pakistan factory fire kills hundreds. Photo: CNN

The toll from the fire, which continued to burn Wednesday, is expected to rise, Commissioner Roshan Ali Shaikh, Karachi's top administrative official, said in a phone interview with CNN affiliate Geo TV.

A child is among the dead, and at least 25 people have been injured, said Zakir Khan of the Karachi Fire Department. All the victims were employees of the factory, he said.

Firefighters were trying to bring the blaze under control, Khan said, but their task was made more difficult because they couldn't enter several portions of the weakened building.

The building might collapse any time because the fire heavily damaged parts of its steel structure, said Mustafa Jamal, a senior government official in Baldia Town, the area of Karachi where the factory is situated.

Dozens of people may be trapped in the basement of the building, according to Khan. An operation is under way to try to rescue them.

Jamal said between 400 and 500 people were in the factory as the fire burned. Several trapped workers jumped from upper floors.

Video broadcast by Geo TV showed large crowds of anxious people outside the smoldering building. And through windows and holes of the factory's scorched outer walls, the footage showed glimpses of flames still flickering within.

Officials say they don't know the cause of the fire.