Brussels will stay at the highest security threat level for another week over fears of an imminent attack, the Belgian government says, as authorities charged a fourth suspect in connection with the terror assaults in Paris.
As Brussels remained under lockdown, the US issued a worldwide travel alert warning American citizens of "increased terrorist threats" in "multiple regions".
France meanwhile launched its first strikes against Islamic State from a newly deployed aircraft carrier in the Mediterranean, as the country ramped up its fight against the jihadist group that has claimed the November 13 gun and suicide bomb attacks.
Separately, French police said they had found a suspected explosives belt in a suburb near Paris where Salah Abdeslam, a key suspect in the attacks and the subject of a massive manhunt in Belgium and France, was known to have been on the night of the atrocities.
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.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
"The threat remains serious and imminent," Michel said, amid fears that the symbolic and institutional capital of Europe could face co-ordinated 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 on 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.
As Brussels remained under lockdown, Washington urged Americans to avoid large crowds and "exercise particular caution during the holiday season".
French police said on Monday they had found a suspected suicide belt with a similar "configuration" to those used in the attacks in a dustbin in Montrouge, south of Paris.
The item, which was found by dustmen 10 days after the attacks in the capital that left 130 people dead, is being analysed "to confirm whether it is explosive", a source close to the inquiry said.
Telephone data placed Belgian-born Abdeslam, 26, in the Montrouge area on the night of the attacks, the source added.
French President Francois Hollande said his country was "at war" with IS following the attacks, and on Monday planes based on the Charles de Gaulle aircraft carrier launched their first attacks against the jihadist group in Iraq and Syria.
As diplomatic efforts to tackle IS gathered pace, Hollande met British Prime Minister David Cameron in Paris in an effort to widen an international alliance against the extremist group.
"We will intensify our strikes, choosing targets that will do the most damage possible to this army of terrorists," Hollande said, adding that NATO allies Britain and France had a "joint obligation" to strike at IS.
Cameron, who earlier laid a wreath at the Bataclan concert venue in Paris where 90 people died, said he supported France's actions and added that "it's my firm conviction that Britain should do so too."
