Belgian prosecutors said on Wednesday four people had been detained after automatic weapons were found in house searches in Brussels as part a counterterrorism investigation overnight.
A fifth was detained in northern France, across the border
State broadcasters said those arrested had links to a group called the Kamikaze Riders, a motor bike group based in the city whose members are mostly of North African origin and previously linked to jihadi groups.
They had plans to carry out an attack, Flemish-language broadcaster VRT said, citing sources, adding those plans were still at a very early stage and no specific target had yet been chosen.
The prosecutors said the detentions were separate from investigations into Islamist attacks in Paris in November 2015, which killed 130 people, and in Brussels four months later in which 32 people were killed.
"During one of the house searches, various weapons were found," the federal prosecutors said in a statement.
Police found Kalashnikovs, bullet-proof vests and ammunition in raids on two garages and four homes in the Brussels' Anderlecht district.
Another person was arrested by anti-terrorist police on the French side of the border with Belgium, a French judicial source said. A French police source said the arrest was part of a joint operation led by the Belgians.
La Voix du Nord newspaper said the arrest in France followed a raid by counterterrorist police in the early hours of Wednesday in a suburb of the northern city of Lille.
The suspect fled the house but was detained in an adjacent street, dressed only in a T-shirt and shorts.