Armed police launched an operation in the Brussels district of Schaerbeek, a day after six suspects were arrested over the suicide attacks on Tuesday.

The Belga news agency reported that two explosions were heard at the start of the operation, with police having sealed off the area. Explosives experts were at the scene, with a bomb disposal robot seen deployed on the platform of a light rail stop.

Public broadcaster RTBF said a man carrying a backpack had been "neutralized" after refusing to obey police orders, but there was no immediate confirmation by officials. A witness claimed officers had approached a person at a transport stop and asked the person to remove their jacket.

RTBF quoted local mayor Bernard Clerfayt as saying a man had been arrested after being shot in the leg.

Connected to Paris raid

A police source was reported as saying that the operation was related to the arrest of suspect Reda Kriket in Paris on Thursday. The 34-year-old was found with heavy weapons and explosives in his apartment.

France's interior minister said that Kriket, who has been wanted since January, was in the "advanced stages" of a plot targeting France.

Visiting Brussels as the operation continued on Friday, US Secretary of State John Kerry said his country would help Belgium with its investigation into the attacks.

Belgian Prime Minister Charles Michel cancelled his attendance at a wreath-laying ceremony with Kerry because of the Friday afternoon police operation.

Belgian prosecutors on Friday said they had released three of the six people who were detained on Thursday. Three individuals - Faycal C., Abou A. and a third unnamed person - were still being held, according to a statement.

Investigators also said they had established through DNA tests that Naijm Laachraoui - who police had been searching for in the wake of Tuesday's attacks - was one of the bombers that blew themselves up at Brussels airport.

German media on Friday reported that a 28-year-old Moroccan man, whose mobile-phone text messages may link him to one of the Brussels bombers, had been arrested.

rc/msh (AFP, AP, Reuters)