The arrests could give investigators new insights into the Islamic State cell believed to have carried out the attacks in both countries.
Authorities announced the arrest of five men on Friday, including Mohamed Abrini, the last identified suspect at large from the Paris attacks. Belgium’s prosecutor’s office said on Saturday that a sixth person had been arrested, but refused to give any further details ahead of an announcement later.
Another one of the six, named only as Osama K by authorities, was identified by Swedish media as Osama Krayem, who is known to have left the Swedish city of Malmo to fight in Syria.
Belgium’s prosecutor’s office confirmed only that Osama K was from Sweden.
Abrini and Krayem are suspected of participating in the two biggest attacks carried out by IS in Europe over the past year, killing 130 people in Paris on November 13 and 32 in Brussels on March 22.
Investigators are trying to determine whether Abrini is the “man in the hat” who escaped the Brussels attacks while three suicide bombers blew themselves up at the airport and in the city’s subway and whose likeness has been broadcast across the media for weeks.
They will also be investigating Krayem’s role in the attack. He was filmed by security cameras at a shopping mall where the bags used by the airport bombers were bought. French authorities also suspect Krayem of having links to the Paris attacks.
The arrests may also help investigators unravel the links between the attacks and IS, the radical Muslim group which straddles Iraq and Syria.
Krayem had earlier been identified posting photos from Syria on social media, according to Magnus Ranstorp, a counter-terrorism expert at the Swedish National Defence College.
“He also tried to recruit people in Malmo,” Mr Ranstorp said.
The detentions were a rare success for Belgian authorities, who have been pilloried for mishandling leads in the investigation. The interior and justice ministers had offered to resign before the detentions.
Despite multiple arrests, Brussels remains under the second-highest terror alert, meaning an attack is considered likely.