More than 100 Belgian police raided multiple addresses around the port city of Antwerp on Tuesday and detained 13 people suspected of links to a jihadist group.
The operation was launched under the authority of anti-terror magistrates as part of a broader investigation, federal prosecutors said in a statement.
“An Antwerp group within the Salafist jihadist milieu has come under scrutiny and the aim of this operation was to further map out the group’s activities,” it said.
“Thirteen people were deprived of their liberty and will appear in the course of the day before the investigating judge, who will decide on a possible arrest.”
The Belgian cities of Brussels and Antwerp have been singled out in the past as alleged rear bases for international jihadist extremism.
Antwerp was the base of the jihad group Sharia4Belgium, which was founded in 2010, and several of its members went on to fight for hardline groups in Syria.
Its former leader, Fouad Belkacem, was sentenced in 2015 in Belgium to 12 years in prison, then stripped of his Belgian nationality in 2018.
Sharia4Belgium has been officially dissolved but Belgian law enforcement fears it could still be a source of inspiration for some Islamists.
In the aftermath of the Paris attacks of 13 November 2015, which left 130 dead, Belgium was accused of being a rear base for international jihadism.
The initial investigation revealed that several of the attackers were from the working class Brussels district of Molenbeek and that the attacks were planned there.
Belgium itself became a target after the arrest in Brussels of Salah Abdeslam, the only surviving member of the Paris attack gang, now on trial in France.
On 22 March 2016, Brussels was hit by a double suicide bomb attack that was claimed by the Islamic State group.
Bombers from the same jihadist cell that attacked Paris blew themselves up at Brussels Zaventem airport and in the metro, killing 32 people and wounding 340.