DHAKA (AFP) - Locked exit gates may to be blame for the high death toll from a fire at a Bangladesh factory that makes sweaters for Swedish clothing brand H&M, which killed 21 people.

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An electrical short-circuit likely triggered the fire at the sweater factory in Gazipur town, but an official probe has been launched into possible breaches of fire safety regulations, local police chief Abdur Rashid told AFP.

"We have heard reports that the gates on the top floor were locked and we are investigating this," Rashid said, adding that most of the victims had suffocated due to smoke inhalation.

The majority of the bodies were recovered from the top floor of the seven-storey Garib and Garib Sweater factory in the industrial town north of the capital Dhaka where the fire started late Thursday, he said.

"Everyone working on the factory's top floor died because the exit gates were locked, they were all women, they were all trapped and they all suffocated," Abdul Momin, a Garib factory employee who said his aunt died in the fire, told AFP.

Several hundred people protested in central Dhaka and Gazipur on Friday.

"This was not an accident, these workers were killed by the factory's blatant disregard for worker safety," head of the National Garment Workers Federation, Amirul Islam Amin, told AFP.

Garib and Garib's website lists current clients including Hennes and Mauritz (H&M), a popular and affordable Swedish fashion brand, and former clients including US retail giants WalMart and JCPenny.

No Garib company representatives could be reached for comment Friday.

The fire started on the second floor of the factory, trapping scores of workers knitting sweaters for export. Police say fourteen women and seven men were killed and around 40 people were injured.

"Three people remain in critical condition in hospital with severe burns and respiratory problems from smoke inhalation," Rashid said, adding that most of the other injured people had been treated for smoke inhalation and released.

He said the casualty number could have been far higher as between 50 and 70 employees were working late after hundreds of other workers had gone home for the day.

The president of the Bangladeshi Garment Manufacturers and Exporters Association said that he believed fire safety regulations were being followed at the factory, and said the deaths were likely due to workers panicking.

"I have heard claims about a locked gate but as an investigation is ongoing. I cannot comment on that now," Abdus Samad Murshedy told AFP.

Fires due to short-circuits and substandard wiring are common in Bangladeshi garment factories, annually killing scores of the country's 2.5 million poorly paid workers who toil 12-14 hours a day in difficult working conditions.

Some 4,500 garment factories accounted for nearly 80 percent of the impoverished country's 16-billion-dollar export income in the last fiscal year.