BRUSSELS (AFP) - Belgian police detained four people for questioning in connection with terrorism after 13 raids on Sunday (March 27) across Brussels and the towns of Duffel and Mechelen to the north, the federal prosecutor said.

"In connection with the terrorism dossier, 13 raids were carried out this morning... in all nine people were questioned, with five of them later released," the prosecutor said in a statement.

The statement made no direct mention of Tuesday's twin bomb attacks on Brussels airport and a metro train in which 28 people were killed and 340 were hurt.

It said there were four raids in Mechelen, about 30km due north of Brussels, and one in Duffel, 40km in the same direction.

The examining magistrate will decide later whether the four people held will be put in preventive detention, it said.

The prosecutor said the authorities would say nothing for the moment about the results of the searches.

The suicide bombings, Belgium's worst ever terror attacks, were claimed by the Islamic State in Iraq and Syria (ISIS).