Several people have died in a fire at a building under construction in Brussels on Tuesday, local authorities have said.

The cause of the fire at the Oxy tower in the centre of the city is unclear, with officials saying at least six people are missing.

While it was put out quickly, when firefighters entered the building they discovered it had spread to the lift shafts, where two lifts were stuck.

They were able to pry one open and found "several victims - we don't yet know exactly how many," said Brecht Speybrouck, spokesperson for the Brussels Labour Prosecutor's Office.

"It could be the six missing people, but there could also be more in the other lift, or elsewhere," he added.

The mayor of Brussels, Philippe Close, thanked the emergency services who "did an exceptional job" in "particularly difficult circumstances."

Prime Minister Bart De Wever arrived at the scene and Philippe, King of the Belgians, is expected later, Belgian outlet RTBF said.

Local media said two people were already taken to hospital with serious burns, while a firefighter was treated at the scene after being exposed to high temperatures.

The Oxy tower is still under construction and will eventually house flats, a hotel, restaurants and bars.

It is located 500m (0.3 miles) from the Grand Place, Brussels' central Baroque square.