Brussels will stay at the highest security threat level for another week over fears of an imminent attack, the Belgian government said Monday

Brussels will stay at the highest security threat level for another week over fears of an imminent attack, the Belgian government said Monday, as authorities charged a fourth suspect in connection with the terror assaults in Paris.

On the third day of an unprecedented security lockdown in Brussels, Belgian Prime Minister Charles Michel said the city would be kept under the maximum level four terror alert for another week, but that schools and the metro system would reopen from Wednesday.

"The threat remains serious and imminent," Michel said, amid fears that the symbolic and institutional capital of Europe could face coordinated Paris-style attacks.

Michel said the army and armed police would remain on the streets in coming days and he advised people to stay away from crowded areas.

The security level will be reviewed again next Monday.

The federal prosecutor's office announced that a man who was arrested during a large police operation in Belgium late Sunday has been charged with involvement in the Paris atrocities.

"He is charged with participating in activities of a terrorist group and with a terrorist attack," the office said in a statement.

The Belgian authorities have now charged four suspects over the Paris carnage.

Two of them, Mohammed Amri, 27, and Hamza Attou, 20, were charged last Monday on suspicion of helping Abdeslam escape to Brussels after the attacks, while a third unnamed person faces charges of aiding him when he reached the city.