BRUSSELS (AFP) - Belgian police on Wednesday (Jan 25) were holding seven people for questioning after raids in the Brussels region as part of an on-going counter-terrorism operation, prosecutors said.

The searches of eight houses were linked to returning extremists from Syria and separate from investigations into the Islamic State in Iraq and Syria (ISIS) attacks in Paris in November 2015 and the bombings in Brussels in March last year, a statement said.

"No weapons or explosives were found. In all, seven persons were arrested and taken for questioning," it said.

Belgium is the EU country with the highest per capita number of fighters who have gone to wage jihad in Syria and Iraq, a figure estimated at 465.

The authorities have tried for years to clamp down on their recruitment, fearing they will return home battle-hardened and even more dangerous.

Belgium has been on high alert since the Paris and Brussels attacks, with investigators believing they were planned and carried out by the same ISIS cell based in the Belgian capital.