PARIS -- Belgian authorities have charged a fourth suspect with terrorism offenses after they detained 16 people on Sunday.
The federal prosecutor said in a statement that the suspect, who was not identified, was charged with "participation in the activities of a terrorist group and a terrorist attack," referring to the Nov. 13 attacks in Paris.
Authorities had charged three others with similar offenses last week.
The other 15 people detained on Sunday evening were released.
Meanwhile, French police say that an explosive belt - without a detonator - has been found in the southern Paris suburb of Montrouge. A police official says the belt was found by a street cleaner on Monday in a pile of rubble. An official said the bomb contained bolts as well as the same explosives used in the Paris attacks.
The belt was found in the same place that fugitive Saleh Abdeslam's cell phone was localized on the day of the attacks.
Belgian police launched new raids in the capital and beyond early Monday in their hunt for a fugitive suspect in the Paris attacks, as Brussels entered a work week grappling with a third day of unprecedented security measures that have closed subways, shopping districts and now schools.
CBS News correspondent Debora Patta reported most of the shops were still shuttered and the metro was shut down as the country woke up to a third day of security lockdown.
Belgian officials said Monday they will keep its alert at the highest level possible in the capital for now and will maintain its security measures to contain a possible attack for at least another week.
Prime Minister Charles Michel told reporters that the rest of the nation would stay at the second-highest level. Even with the alert level this high, Michel said that schools and the subway system would reopen progressively as of Wednesday.
"We do our utmost to keep the situation under control," Michel said after a meeting of the national security council.
In Paris, British Prime Minister David Cameron said he will ask for parliamentary approval for the U.K. to join airstrikes against Islamic State of Iraq and Syria (ISIS) extremists in Syria. If approved, Britain would join other nations carrying out bombings in Syria, including the U.S., France and Russia.
France's Defense Ministry said it had launched its first airstrikes from the Charles de Gaulle aircraft carrier, bombing ISIS targets in the Iraqi cities of Ramadi and Mosul in a seven-hour operation. The ministry said that four Rafale fighter jets were sent from the carrier on Monday afternoon, with two each flying over each city. France has already carried out strikes against ISIS targets in Syria.
The raids in Belgium began late Sunday, capping a tense weekend that saw hundreds of troops patrolling streets and authorities hunting for one or more suspected extremists, including Salah Abdeslam, a fugitive since being named a suspect in the Paris attacks that killed 130 and wounded hundreds of others on Nov. 13. Between Sunday night and midday Monday, 21 people were detained.
Abdeslam's brother Mohammed appealed on local television, urging Salah to hand himself over to police.
"I think that effectively at the last minute Salah decided to change course," he said. "He perhaps saw or heard something, and decided not to carry out his plans."
The Belgian government kept the capital on the highest state of alert in the face of what it described as a "serious and imminent" threat, preventing a return to normal in the city that is also host to the European Union's main institutions.
Federal prosecutor Eric Van Der Sypt said 19 raids were carried out Sunday in Molenbeek, home to many of the Paris attackers, and other boroughs of Brussels, and three raids were carried out in Charleroi. Abdeslam was not among those arrested. Van Der Sypt said no firearms or explosives were found.
Belgian Interior Minister Jan Jambon told the RTL network that Abdeslam "must have a lot of support on our territory. That's why all these searches being conducted at the moment are important."
Police fired two shots at a car that approached them as they searched a snack bar in Molenbeek, Van Der Sypt said. The vehicle escaped but was stopped later in Brussels, and a wounded person inside was arrested. It wasn't immediately known if the person was linked to the investigation into the Paris attacks.
Cameron and French President Francois Hollande, meeting in Paris, paid a quiet visit to the Bataclan concert venue, which saw the worst of the carnage in Paris. Hollande plans to meet Tuesday with President Barack Obama, then Wednesday with German Chancellor Angela Merkel and with Russian President Vladimir Putin on Thursday in pursuit of a unified strategy in Syria.
Britain has been carrying out airstrikes in Iraq, but Cameron has long wished for an expanded mandate to include targets in Syria. But having lost a vote once two years ago, his government had been reluctant to even suggest a vote until it could be certain it would win.
Belgian authorities said more raids Monday in Brussels and eastern Liege yielded five more detentions.
The federal prosecutor said in a statement that a BMW sighting near Liege that had been rumored to contain Abdeslam had no links with the ongoing investigation.
Frank Foley, lecturer in war studies at King's College London and a terrorism expert, said it was difficult to know if what Belgium was doing was justified because authorities have provided so few details about their decision.
Foley said the measures intended to avoid attacks similar to the ones that caused devastating carnage in Paris could even be counterproductive if they last too long.
"If these dramatic measures continue in Brussels, we will be doing the terrorists' job for them," he said. "The government may be unintentionally contributing to the atmosphere of fear."
But Henry Willis, director of RAND Homeland Security and Defense Center, likened the clampdown in Paris to the reaction of U.S. authorities after the Boston marathon bombing.
"They did shut down the city for a couple of days and when they lifted those restrictions, that's when they caught the terrorist," he said.
Several of the Paris attackers had lived in Brussels, including Abdelhamid Abaaoud, the man who authorities say orchestrated the plot. He was killed Wednesday in a standoff with French police.
French police issued a new public appeal for help in identifying the third attacker who was killed in the assault at the national stadium. On Sunday, they posted a photo of the man on Twitter, asking the public for information.
Greek police confirmed that the man posed as an asylum seeker before the carnage. Public Order Minister Nikos Toskas said the man traveled to the island of Leros on Oct. 3, but he gave no further details.
Two senior Greek law enforcement officials told The Associated Press that the man traveled with another attacker identified as Ahmad Al Mohammad. Both men were rescued by the Greek coast guard while traveling from nearby Turkey on a boat carrying nearly 200 migrants and refugees that sank before reaching Greece. The officials requested anonymity because they weren't authorized to speak to the media.
France has extended a state of emergency which allows police raids, searches and house arrest without permission from a judge, for three months. On Saturday, it also extended a ban on demonstrations and other gatherings through Nov. 30, when a U.N. climate conference with more than 100 heads of state is scheduled to start.
ISIS has claimed responsibility for Paris attacks; the suicide bombings in Beirut that killed 43 people and injured more than 200; and the downing of the Russian jetliner carrying 224 people in Egypt's Sinai Peninsula. All happened within the past month.