Belgian police on Sunday arrested four youths over messages they exchanged allegedly plotting a jihadist attack deemed "imminent enough to intervene", a spokesman for the federal prosecutor's office told AFP.

The four -- three minors in their "late teens" and an adult man aged 18 -- were arrested when police raided home addresses in the cities of Brussels, Ninove, Charleroi and Liege, the spokesman, Eric Van Der Sypt, said.

No weapons or explosives were found. Police took away mobile phones and laptops for analysis, he said, confirming reports from Belgian media RTBF and HLN.

The messages the four exchanged on the internet about the alleged planned attack were "worrying enough to intervene, to do house searches," Van Der Sypt said.

"It's not that they were planning something tomorrow, but still imminent enough to intervene," he said.

The relative youth of the suspects heightened authorities' wariness.

"They're very flexible. If somebody gives them a weapon, for example, things can go very, very fast. So we don't want to take any risk," the spokesman said.

"From the moment that we think that we have to intervene, even if we have not too much (evidence) afterwards, we prefer intervening in time."

Police were questioning the four suspects and an investigating judge was on Sunday or Monday to determine what would ensue, he said.

Belgian authorities remain highly vigilant since the 2016 jihadist attacks by suicide bombers that killed 32 people in blasts at 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 fatally shot by police.

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