Belgian police arrested 21 more people in raids in and around Brussels by Monday afternoon, searching for those behind the deadly attacks in Paris 10 days ago, while the city remained in lockdown for a third day.
Belgium announced terrorism charges related to the attacks against an unidentified fourth suspect, who was among 16 people detained Sunday. The 15 others arrested were released.
Belgian Prime Minister Charles Michel said Monday that Brussels will maintain its highest security alert level, which will be reviewed again next week. Schools and the metro system are set to re-open on Wednesday, he added.
"We are still confronted with the threat we were facing yesterday," Michel told a news conference.
Separately, police said an explosive vest was found in the southern Paris suburb of Montrouge. The explosives were the same type used in the Paris attacks, according to the Associated Press.
Salah Abdeslam, the 26-year-old suspect from Brussels who has been on the run since his elder brother blew himself up at a Paris cafe is still on the run. A third brother, who was not involved, said Abdeslam may have thought better of going through with the killing. Belgian police fear he returned home to launch new attacks.
Shops, colleges and offices in Brussels, which is also home to the headquarters of the European Union and NATO, remained closed Monday after the prime minister warned of imminent Paris-style attacks.
"What we fear is an attack similar to the one in Paris, with several individuals who could possibly launch several attacks at the same time in multiple locations," Prime Minister Charles Michel told a news conference early on Sunday evening.
Belgium has been at the heart of investigations into the Paris attacks in which 130 people were killed. Two of the suicide bombers, Brahim Abdeslam and Bilal Hadfi, lived in Brussels.
Forced indoors, some of Brussels' multilingual 1.2 million residents took to social media to share and joke about their frustration. Twitter hashtag #BrusselsLockdown spawned many photos of kittens, some in combat gear, a wry reference to the security level: Four, or in French, Quatre - pronounced Cat'.
U.K. Prime Minister David Cameron is pledging to bolster Britain's efforts to combat terrorism by increasing defense spending by an extra £12 billion ($18.2 billion) over the next 10 years.
Cameron will set out a five-year defense and security plan to parliament on Monday that will include nine Boeing P-8 Poseidon maritime patrol aircraft, two 5,000-strong "strike brigades" and extending the life of its Typhoon fighter jets by 10 years.
"This government has taken a clear decision to invest in our security and safeguard our prosperity," Cameron wrote in the foreword to the review, published by his office.
"So while every government must choose how to spend the money it has available, every penny of which is hard-earned by taxpayers, this government has taken a clear decision to invest in our security and safeguard our prosperity."
Meanwhile, on a trip to visit President Francois Hollande in Paris, Cameron told a joint press conference with the French leader that the U.K. should carry out military airstrikes alongside France and other partners in Syria against ISIS.
The U.K. leader also said he had offered France use of a British airbase in Cyprus as well as additional assistance with air-to-air refueling.
"I firmly support the action that President Hollande has taken to strike ISIL in Syria and it is my firm conviction that Britain should do so too," he said.
Hollande said France planned to intensify its airstrikes on ISIS in Syria with its only aircraft carrier, the Charles de Gaulle, getting into position on Monday for strikes.
— Reuters and the Associated Press contributed to this report