Belgian police have carried out a major operation in the Molenbeek district of the capital Brussels which was home to many of the jihadis who carried out deadly attacks in Paris and on Brussels airport.

Officials did not say who police, who sealed off several roads in the immigrant area and who were backed by a helicopter flying overhead, were targeting in the raids.

"Several raids are being carried out as part of an investigation into terrorist activities," Brussels prosecutor's office spokeswoman Céline Praille told RTL radio.

Local media said several individuals had been arrested in the raids that were ongoing on Saturday evening.