Three of the suspects were arrested "outside the door of the federal prosecutor's office," a spokesperson said. One was detained in Jette, on the outskirts of the city, while two others were arrested in Brussels, the official added.

No other information about the individuals was given.

Home to the headquarters of the European Union and NATO, Brussels is still reeling from Tuesday's terror attack, when bombs were detonated at the city's international airport and a metro station. The attack has since been claimed by the so-called "Islamic State" (IS).

Show of support

US Secretary of State John Kerry landed in the Belgian capital on Friday in a show of support from Washington.

Tuesday's attacks came just four months after 130 people were killed in a series of bombings and shootings in Paris.

Highlighting the threat from extremists, French Interior Minister Bernard Cazeneuve said Thursday that police had detained a suspect in the Paris area who was "in the advanced states" of plotting an attack against France.

The suspect, a French national, was not linked to the Paris or Brussels attacks, but Cazeneuve said the man "belongs to a terrorist network that sought to strike our country."

Known to authorities

Meanwhile, the brothers who carried out the bomb attack at Brussels Airport and the metro station were known to US authorities and listed on US terrorism databases, the American television network NBC reported Thursday.

Citing two unnamed US officials, the report says Ibrahim and Khalid El Bakraoui were identified as a "potential terror threat" on US databases, but they would not specify on "which of the many US terrorism databases the brothers were listed."

Prior to Tuesday's attacks, the Belgian brothers had a string of criminal convictions to their names related to carjackings, robberies and shootouts with police.

A huge manhunt is still underway for a third attacker, whose bomb failed to detonate at Brussels Airport, a man seen wearing a hat on security footage (below).

Resignation offers

Also on Thursday, Belgium’s interior and justice ministers offered their resignation over a failure to track Ibrahim El Bakraoui, who was expelled from Turkey as a suspected fighter and who blew himself up at Brussels’ International Airport on Tuesday.

Interior Minister Jan Jambon and Justice Minister Koen Geens tendered their resignations to Prime Minister Charles Michel, who in turn asked them to stay on.

"In time of war, you cannot leave the field," Jambon, a center-right Flemish nationalist, said.