Brussels police were searching for two gunmen Wednesday after shots were fired outside a metro station, with authorities saying there was "no indication of a terrorist motive".

A police spokeswoman said shots were heard at around 6:00 am outside a station close to the city centre.

Surveillance footage obtained by broadcaster RTBF showed two masked people carrying automatic rifles outside the metro, with at least one letting off repeated shots.

"CCTV footage shows two people, at least one of whom is armed, the shots were fired into the air, there are no injuries," the police spokeswoman told AFP. 

The suspects fled into the metro and entered tunnels running under one of the city's main railway stations, she added.

"A major search operation was launched after the available video surveillance images showed two suspects fleeing into the metro tunnels," prosecutors said in a statement.

"At this stage, there is no indication of a terrorist motive for this shooting."

Brussels transport authorities said that three metro stations had been closed on the orders of the police and traffic on two lines interrupted.  

Brussels has seen an increase in gun crime linked to disputes between rival drug gangs in the city.

Local media citing police figures say that there were 89 shooting incidents, killing nine people, in the city last year.