Belgian police raided homes in Brussels on Tuesday and detained 10 people on suspicion of operating a recruitment ring for ISIS.
"Our investigation points to several persons having left for Syria to join Islamic State," Belgium's federal prosecutors said in a statement.
Prosecutors said they had seized computer equipment and mobile phones.
10 people were taken in to be questioned by the anti-terrorism branch of the Liege police.
Belgium has one of the highest per capita rates of participation in militant groups such as ISIS (also known as the Islamic State, ISIL or Daesh) in Syria and Iraq.
The federal prosecutor's office said the raids were not linked to the Paris attacks.