A number of workers found themselves trapped in a lift as the blaze spread through the lift shaft, reaching several floors of the site, known in the Belgian capital as the Oxy Tower.

"Six bodies have been found," a spokesman for Brussels Mayor Philippe Close told AFP.

Authorities had initially given a toll of two to three people killed at the building, which is situated in central Brussels.

Two badly wounded victims were taken to hospital.

The six bodies recovered must still be identified, authorities said.

The cause of the fire is still unknown.

Labour inspectors will investigate whether the site complied with regulations, said Valentina Marocchi of the public prosecutor's office.

More than 200 workers are employed at the site.

The fire broke out on the second floor of the building before spreading to the first floor as well as to the second basement level via at least one elevator shaft, a fire department spokesperson told AFP.

The fire was extinguished in the early afternoon, but civil protection units were called in as reinforcements to try to free trapped victims.

Belgium's King Philippe and Prime Minister Bart De Wever visited the scene and met with rescuers.

Belgian Interior Minister Bernard Quintin said he was "horrified" by the tragedy.