Authorities have raided dozens of homes and arrested 12 suspects in a major anti-terror investigation which they say required "immediate intervention."

The federal prosecutor's office said early Saturday homes and car ports were searched in 16 municipalities, mostly in and around Brussels.

The statement said 40 people were taken for interrogation, of which 12 were arrested. It said there were no major incidents during the raids.

The operation comes as Belgium is still under the second highest terror alert in wake of the March 22 attacks on the Brussels subway and airport which left 32 people dead.