Antwerp police carried out five searches in Merksem, Borgerhout, Deurne, Sint-Jans-Molenbeek and Eupen on Monday night at the request of an investigating judge.

The prosecutor's office said five people were arrested.

"At least two of the people involved are suspected of planning to carry out a terrorist attack in Belgium. The target of the attack has not yet been determined," prosecutors said.

Meanwhile, Brussels police carried out raids in the nearby localities of Zaventem, Molenbeek-Saint-Jean and Schaerbeek as part of a separate case and arrested three people.

"These people are also suspected of planning to carry out a terrorist attack in Belgium," the office said. "There are links between the two cases but further investigation will have to reveal the extent to which the two cases were intertwined."

Belgian broadcaster RTBF reported that the Brussels and Antwerp cases initially focused on two young adults suspected of violent radicalism and that investigations revealed links between the two, with potentially dangerous individuals gravitating in their entourage.

The arrests came as suspected members of a cell that carried out the deadliest peacetime attacks on Belgian soil seven years ago are on trial in Belgium.

The defendants face charges including murder, attempted murder and membership, or participation, in the acts of a terrorist group, over the morning rush hour attacks at Belgium's main airport and on the central commuter line on March 22, 2016.

As well as the 32 people who died in Brussels that day, about 900 were injured or suffered mental trauma.

Among the accused is Salah Abdeslam, the only survivor among the so-called Islamic State extremists who in 2015 struck the Bataclan theatre in Paris, city cafes and France's national stadium.

He was sentenced to life in prison without parole over the atrocities in the French capital.

Australian Associated Press