Belgium's top terror threat analyst says around 640 Belgians have fought or trained in Syria and that the country is to remain on high alert.

Paul Van Tigchelt, head of Belgium's terror threat analysis center, told RTL radio Thursday that some 160 Belgians are believed to still be in Syria, with a further 110 thought to have died there.

Belgium has been on its second highest threat level — level three — for more than a year, with soldiers and extra police deployed to guard important public buildings and gatherings.

Van Tigchelt said "an attack is still possible. There is no reason for people to be scared, but we remain at level three."

Thirty-two people were killed on March 22 in suicide bomb attacks on the Brussels airport and subway.