At least 280 people have been killed in a fire that ripped through a packed garment factory in the Pakistani port city of Karachi.

The fire engulfed the four-storey factory, where 1,500 people are believed to have worked, in a matter of minutes on Tuesday evening (local time).

The death toll is expected to rise as rescue teams continue to try to gain access to some parts of the building.

Hundreds of bodies have so far been pulled from the smouldering remains of the building and dozens of other people were injured when they were forced to jump out of windows to save their lives.

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Pakistani officials say many of the workers were trapped as the factory had only one exit and that its windows were blocked by metal bars.

The blaze came shortly after another at a shoe plant in Lahore that killed at least 21 people.

Karachi fire chief Ehtesham Salim said the garment factory was a fire hazard.

"The factory was flimsily built. It was packed like a box with little room left for ventilation. There were no emergency exits," he said.

Mr Salim said about 35 fire engines took part in the emergency at the factory, which workers said made underwear and also plastic utensils.

He added that the blaze was Karachi's "biggest fire in terms of deaths in decades".

"Our firemen are searching every nook and corner of the factory despite having limited resources to cope with such a grave situation," he said.

Police are now looking for the factory's owners.

Relatives of victims mourn at the site of the Karachi factory fire. ( AFP: Rizwan Tabassum )

Shouting and sobbing relatives of trapped workers, desperate to get inside the building, scuffled with police during the night, witnesses said.

Rescuers used arc lights to work through the night. A steady stream of bodies were stretchered out, covered by white sheets.

Abdus Salam, a doctor at Karachi's civil hospital, said 10 women were among the dead.

"The bodies are badly charred," he said, adding that at least 65 other workers had suffered broken bones after jumping out of windows to escape the fire.

Firefighters on crane lifts reached through the gutted building's windows to rescue some trapped survivors, who were taken to local hospitals suffering from burns and smoke inhalation.

Mohammad Saleem, 32, who broke a leg after jumping out of the second floor, said he and his colleagues were hard at work late when flames suddenly reached their section.

"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," he said.

"I fell on the ground and it was extremely painful.

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

Firefighters carry an employee from the shoe factory in Lahore. ( AFP: Arif Ali )

Negligence

Officials said the cause of the Karachi fire was not yet known, but Rauf Siddiqi, the industry minister for Sindh province, said the factory 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," Mr Siddiqi said.

Earlier, the blaze in Lahore trapped dozens of workers in the shoe-making factory, killing 21 and injuring 14 others, local officials and medics said.

Tariq Zaman, a government official, said the fire was caused by a faulty electricity generator.

Karamat Ali, a spokesman for the local government's emergency rescue team, said fire crews succeeded in bringing the blaze under control after several hours and rescue workers had evacuated all those trapped inside.

The office of prime minister Raja Pervez Ashraf said he "expressed shock and grief over the loss of precious lives and directed the concerned authorities to provide best medical treatment to the injured".

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

According to central bank data, the textiles industry contributed to 7.4 per cent of 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.

ABC/AFP