MORE than 310 people have died in fires that gutted factories in Pakistan's two largest cities, in tragedies that prompted calls for an overhaul of poor industrial safety standards, officials say.

At least 280 people died at a garment factory in Karachi, in the worst blaze in decades to hit Pakistan's biggest city, just hours after 21 died at a shoe factory in Lahore, close to the Indian border.

Dozens of others were hurt in Karachi as they jumped out of windows from the four-storey building to escape the blaze that began on Tuesday night in a bid to save their lives, as sobbing relatives of trapped workers scuffled with police overnight.

"The death toll is 289. This is not final, search for more bodies continues," the city's top administration official, Karachi Roshan Shaikh, told AFP as more victims were recovered.

City police chief Iqbal Mehmood had earlier put the toll at 240.

Karachi fire chief Ehtesham Salim said rescue workers were finding large groups of bodies on the lower floors of the factory.

"Our firefighters are finding bodies in greater numbers from the lower floors of the factory," he said.

"We didn't find bodies in ones or twos, but in the dozens, which is why the death toll is increasing so alarmingly," he added.

Salim said the fire probably originated on the ground floor, giving those workers in the basement and on that level less time to escape.

Abdus Salam, a doctor at Karachi's Civil Hospital, said at least 65 workers suffered broken bones after jumping out of windows.

Pakistani Interior Minister Rehman Malik said he had ordered an inquiry into both fires, as officials said the factory in Karachi in particular had been flimsily built, lacked emergency exits and had developed cracks in the walls.

"It was packed like a box with little room left for ventilation. There were no emergency exits," Salim said.

According to workers, the factory produced underwear and plastic utensils.

Salim said the disaster was Karachi's "biggest fire in terms of deaths in decades".

In January 2009, 40 people were killed, more than half of them children, when a fire engulfed dozens of wooden homes in Karachi's impoverished Baldia neighbourhood.

The garment trade is vital to Pakistan's shaky economy.

According to central bank data, the textiles industry contributed 7.4 per cent to Pakistan's GDP in 2011 and employed 38 per cent of the manufacturing sector workforce. It accounted for 55.6 per cent of total exports.

Noman Ahmed, from the NED University of Engineering and Technology in Karachi, said few industries and businesses implement the law on safety and fire exits, finding it easy to avoid because of lack of effective monitoring.

"Most of our shopping centres and markets too have no safety mechanism, which the authorities should review seriously, otherwise it could cause graver tragedies in future," he said.

Mohammad Saleem, 32, who broke a leg after jumping out of the second floor, said he and his colleagues were hard at work late on Tuesday.

"It was terrible, suddenly the entire floor filled with fire and smoke and the heat was so intense that we rushed towards the windows, broke its steel grille and glass and jumped out," Saleem told AFP.

"It was extremely painful. I saw many people jumping out of windows and crying in pain for help."

Around 150 employees were working at the time in one of the factory's three round-the-clock shifts, Saleem said.

Officials said the cause of the fire was unknown but Rauf Siddiqi, the industry minister for the southern province of Sindh of which Karachi is the capital, said the owner was under investigation for negligence.

"We have ordered an inquiry into how the fire erupted and why proper emergency exits were not provided at the factory so that the workers could escape," Siddiqi said.

In Lahore, flames also trapped dozens of workers in a shoe-making factory, killing 21 and injuring 14 others, local officials and medics said.

Tariq Zaman, a government official, blamed the blaze on a faulty generator.

Originally published as Pakistan factory fires kill 310: officials