State-run Belgian broadcaster RTBF says four suspected extremists detained by police were collecting money to buy arms and explosives and developing plans to strike at a crowded target like Antwerp's main train station.

Belgian prosecutors have said all four suspects have been charged with participating in the activities of a terrorist group, but declined to give additional details.

RTBF said Thursday the suspects were exchanging encrypted messages with other Belgians in Syria including Hicham Chaib, a leader of the Islamic State extremist group in its Syrian stronghold of Raqqa.

The broadcast says it was those communications that worried Belgian authorities, who detained the suspects Wednesday as well as a number of minors following searches in the Antwerp area.

The broadcaster said one suspect and one minor are still being held.