Belgium has placed Brussels at the highest terror alert level, citing a "serious and imminent threat that requires taking specific security measures as well as specific recommendations for the population."

The announcement by the Crisis Centre of the Belgian Interior Ministry is advising the public to avoid places where large groups gather -- such as concerts, sporting events, airports and train stations -- and comply with security checks.

"Following our latest evaluation... the centre has raised its terror alert to level 4, signifying a very serious threat, for the Brussels region," said a statement from OCAM national crisis centre.

The increase in alert level for Brussels comes as authorities investigating last week's terror attacks in Paris conduct raids in Belgium as they work to identify and take down the network of terrorists behind the carnage.

Salah Abdeslam, 26, is the subject of an international search warrant. He was last seen driving toward the Belgian border when police stopped and questioned him a few hours after the attacks, not knowing that he was allegedly involved. His whereabouts are unknown.

Hours before the alert, Belgian authorities charged a third person in connection to the terrorist attacks in Paris.

This is a developing story, check back for updates.