Belgian authorities launched six raids on Thursday in Brussels linked to a suicide bomber who allegedly took part in the recent attacks in Paris.
The operation was taking place in Molenbeek and Jette in the Brussels region.
Bilal Hadfi, 20, has been identified as one of three suicide attackers at the Stade de France. The raids are centring on "his entourage", a Belgian official told the Associated Press news agency.
Investigators say Hadfi was a French national who was living in Belgium and had spent time in Syria.
At least 129 people were killed in the attacks in six different locations across Paris last Friday.
Al Jazeera’s Paul Brennan, reporting from Brussels, said there has been one reported arrest in the operation so far.
"The raids are combination of old radicalisation issues being cleaned up now and ones related to Hadfi, the Paris suicide bomber," Brennan said.
Earlier on Thursday, Belgian Prime Minister Charles Michel rejected criticism over the role security services played before the Paris attacks, which French authorities say were prepared and organised in Belgium.
"I do not accept the criticisms which is aimed at denigrating the work of our security services," Michel said in a speech to parliament.
He added his country will spend an extra 400 million euros ($427m) in the fight against security threats within Belgium.
"In addition to the extra funding pledged, the Belgian prime minister also promised to increase the number of intelligence officials, and tackle hate-preaching - the messages of radicalisation," Al Jazeera's Brennen said.
Source: Al Jazeera and agencies