Belgian authorities charge fourth suspect with Paris attacks

A Belgian anti-terrorism judge on Monday charged a suspect with involvement in the Paris attacks, the federal prosecutor's office said, the fourth person to face charges in Belgium over the 13 November atrocities.

The suspect was arrested in a series of police raids in Belgium on Sunday night along with 15 others who were all released without charge, as authorities warned of a Paris-style plot facing Brussels which was on security lockdown on Monday.

Prime Minister Charles Michel announced on Monday night that schools and the metro would reopen in the city on Wednesday after earlier warning that the city faced a "serious and imminent" threat of attacks similar to those that killed 130 people in France.

Meanwhile in the southern Paris suburb of Montrouge, a street cleaner reportedly found an explosive belt without detonator in a pile of rubble, French police told the Associated Press. Police are analysing the belt to see if it may be linked to the Paris attacks.

In Brussels, the unnamed fourth suspect was charged late Monday with "participating in activities of a terrorist group and with terrorist attack (Paris)," the prosecutor's office said in a statement. 

"After thorough interrogation by the federal judicial police, the remaining 15 persons were released by the investigating judge."

Mohammed Amri, 27, and Hamza Attou, 20, were charged last Monday on suspicion of helping alleged Paris attacker Salah Abdeslam with escaping to Brussels after the attacks in which 130 people died.

A third person who has not been publicly named was also charged with involvement in the Paris attacks on Friday and is reported to have helped Abdeslam when he was dropped off in the Belgian capital.

The charges come after more than 20 raids around the country over Sunday night and into Monday. Key suspect Abdeslam, however, slipped through the net.

"Salah Abdeslam was not caught during the raids," federal prosecutor spokesman Eric Van Der Sypt told a press conference just after midnight, meaning Europe's most wanted man remains at large.

Abdeslam's elder brother Ibrahim blew himself up outside a Paris bar in the 13 November attacks.

Armed officers and troops have been patrolling the nearly deserted streets of the tense Belgian capital all weekend after the government raised the terror alert to the highest level of four in the city.

The European Union and NATO, which are both based in Brussels, said they would increase security and urged non-essential staff to work from home on Monday.