The three minors, aged 15 to 17, are not thought to be implicated directly in what was considered a looming attack on a Brussels concert hall, but allegedly have espoused extremist Islamist beliefs, a source told AFP, confirming an online report by France’s Journal du Dimanche newspaper.

Belgian police on Sunday detained three minors in their “late teens” and an 18-year-old for what prosecutors said were messages plotting an attack deemed “imminent enough to intervene”.

A source close to the investigation told AFP the three minors in particular were targeting the Botanique cultural complex, one of the capital’s best-known sites.

The initial investigation indicates the adult suspect was planning a separate attack, and was in contact with one of the three minors.

The arrests stemmed from a police operation looking into people deemed potentially violent and with links to Islamic extremism.

The suspects were arrested in raids on home addresses in the cities of Brussels, Ninove, Charleroi and Liege.

No weapons or explosives were found. Police took away mobile phones and laptops for analysis.

Belgian authorities remain highly vigilant since the 2016 jihadist attacks by suicide bombers that killed more than 30 people in blasts at the Brussels airport and the city’s metro system.

And in October last year, a Tunisian man shot dead two Swedish football fans in Brussels before being shot and killed by police.