Belgian police on Saturday arrested four people on suspicion they planned to go to Syria and Libya, the federal prosecutor's office said, as the nation remains on high alert following the Paris attacks.

Officials believe around 500 Belgians may have gone as foreign fighters to Syria, joining up with militants from the Islamic State (ISIS) group which claimed the November attacks on the French capital.

And with Libya in turmoil since the 2011 revolution, there are growing fears it could provide fertile ground for ISIS recruitment in a region much closer to Europe.

A spokesman for the federal prosecutor's office said police arrested the four in Jumet, near the airport in the southern city of Charleroi, because they were "possibly planning to go to Syria or Libya."

"A judge will decide tomorrow whether to detain them further," he said.

The arrests were not linked to the ongoing investigation into Paris attacks, he said.

Belgian police have arrested 11 people in connection with the November 13 attacks in Paris which left 130 people dead and hundreds wounded since it emerged that the violence was largely organised and coordinated from Belgium.

Earlier this month, the authorities identified three safe houses used by key suspects -- in Brussels, in Charleroi, an hour's drive south of the capital, and in Auvelais, a village near the French border.