(Adds inquiry, compensation)

DHAKA, Dec 9 (Reuters) - At least four workers were killed and about 50 others were injured when the roof of a 22-storey building collapsed as it was being demolished in the Bangladeshi capital Dhaka, police and witnesses said on Sunday.

An unspecified number of workers were also believed to be trapped inside the Rangs Building, which was one of the tallest in Bangladesh's capital. The collapse happened on Saturday night.

"It came down with a roaring sound," said one survivor.

Army personnel, along with police and firefighters, were working to rescue the labourers trapped inside.

"Rescuers arrived shortly after the accident but it was difficult to get in," said a fire brigade officer.

Authorities had ordered the demolition of the building in the centre of Dhaka because its construction violated rules, officials said.

City development authorities had opted to demolish the building by hand rather than with explosives, providing temporary jobs to hundreds of workers in the city with high unemployment. Demolition work began months ago.

The government on Sunday formed a committee to investigate the cause of the collapse and offered to pay compensation to the families of the victims, said Mainul Husein, government adviser for housing and public works.

The rescue effort had been suspended to avoid the risk of a further accident, a senior fire fighting official said.

"We are not allowed to use drill machines as experts apprehend that may cause the entire building to collapse," Selim Nawaz Bhuyian, a deputy director of the Fire Service, told Reuters.

Others said the hopes for finding anyone trapped alive was fading fast.

(Reporting by Serajul Islam Quadir; Writing by Anis Ahmed; Editing by Matthew Tostevin)

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