The death toll from the collapse of a factory complex in Bangladesh passed 500 Friday as the prime minister said Western retailers had to share some responsibility for the plight of garment workers.

It also emerged that an engineer who had warned that the building may be unsafe before it imploded on April 24 was being questioned by police after becoming the latest person to be arrested over the disaster.

With bulldozers clawing away at the mountain of rubble at the site, the number of bodies being recovered from the country's deadliest industrial disaster has been increasing sharply.

Lieutenant Mir Rabbi, an officer in a special army control room set up to coordinate the rescue operation, told AFP the "death toll now stands at 501", a sharp rise on the figure of 441 compiled by authorities on Thursday evening.

Dozens more people are thought to have been buried alive after the eight-storey building collapsed in Savar, which lies around 30 kilometres (20 miles) to the northwest of Dhaka.

Around 3,000 garment workers were on shift at the time of the disaster in the Rana Plaza compound, which housed five different textile factories.

Spain's Mango, Britain's low-cost Primark chain and the Italian label Benetton were among the retailers who have confirmed having products made at Rana Plaza where the typical worker took home less than 40 dollars a month.

The collapse was the latest in a series of disasters to befall the $20 billion industry, which accounts for 80 percent of the country's exports.

A fire at another factory compound killed 111 workers last November and witnesses say the latest disaster happened after bosses insisted staff remain at their workstations even though cracks had been detected in the building.

In an interview with CNN, Bangladeshi Prime Minister Sheikh Hasina defended the industry's safety record, saying the recent deadly explosion at a fertiliser plant in the United States showed that no country was immune.

"Anywhere in the world, any accident can take place," she said.

Some Western fashion brands have said they are considering their futures in Bangladesh and Disney has already announced it is pulling out of the country.

The prime minister insisted that "Bangladesh now is a place for good conditions for the investment" but she also suggested that Western firms drawn to the country by the cheap labour costs could hike salaries.

"If they want to do business, these buyers, they also should also consider increase the prices of the garments so that the business can run properly and labour can get a good salary so they are also partly responsible for it," she said.

"What I is feel is that all the investors when they come here they get cheap labour and that's why they come here."

Industry bosses are desperate to avoid others following the lead of Disney in pulling out of the country and have promised to come up with credible answers to concerns raised about factory safety.

At least 12 people have been arrested over the disaster, including the owner of the Rana Plaza compound. Civil engineer Abdur Razzaq Khan was detained on Thursday night after police said he had given the building the all-clear on April 23 after inspecting the cracks.

Khan was quoted as telling one Bengali newspaper, Jugantor, on the night before the tragedy that the building was "risky" and should be the subject of "further investigation".

But Dhaka police's deputy chief A.B.M Masud Hossain said that investigators had found that Khan had cleared the building to carry on operating while at the same time suggesting that the owner seek further advice.

"According to our information he told the building owner, other owners (of shops and other businesses based in the building) and journalists that these cracks were not a major problem at this time," Hossain told AFP.

The disaster has led to a slight thaw in the usually icy relations between Hasina and her long-time rival Khaleda Zia, leader of the main opposition Bangladesh Nationalist Party (BNP).

After Zia accepted a request from the government to back down on her calls for a nationwide strike scheduled for May 2, Hasina has now asked her arch-rival to talks in a bid to end a dispute over upcoming elections.