Brussels will stay at the highest security threat level for another week over fears of an imminent terror attack, the Belgian government said, as the United States issued a worldwide travel alert for its citizens.

Authorities in Belgium and France were hunting for Belgian-born Salah Abdeslam, a key suspect in the Paris attacks on November 13, when gunmen and suicide bombers killed at least 130 people.

French police were on Tuesday analysing a suspected suicide belt similar to those used in the Paris assault, according to sources close to the investigation.

Authorities found the object -- which lacked a detonator -- in a dustbin in the southern suburb of Montrouge. Telephone data placed Abdeslam in the area on the night of November 13.

Another police source said the belt appeared to have "the same configuration" as those used by the jihadists.

Fearing a similar attack, Belgium maintained an unprecedented security lockdown in Europe's capital Brussels, with Prime Minister Charles Michel warning Monday that the threat "remains serious and imminent".

Belgian authorities have charged a fourth person in connection with the bloodshed in Paris, which was claimed by the Islamic State group.

Washington and Paris have stepped up their fight against IS, with France launching its first strikes from a newly deployed aircraft carrier in the Mediterranean and the United States calling for more international cooperation against the group.

Underlining heightened global fears of attacks after Islamists killed scores in Mali, Turkey, Lebanon and Nigeria in recent weeks, the US government issued a worldwide travel alert warning American citizens of "increased terrorist threats".

"Current information suggests that ISIL (another acronym for Islamic State), Al-Qaeda, Boko Haram and other terrorist groups continue to plan terrorist attacks in multiple regions," said a State Department travel advisory.

US President Barack Obama was due to meet French leader Francois Hollande in Washington on Tuesday.

The French president will also hold talks with Germany's Angela Merkel on Wednesday, Russia's Vladimir Putin on Thursday and Chinese President Xi Jinping on Sunday, as world leaders rally support for a global fight to crush IS.

The UN Security Council on Friday authorised "all necessary measures" to fight the group.

- Lockdown -

In Brussels, an eerie atmosphere hung over the city with soldiers in camouflage patrolling everywhere, from railway stations to EU institutions.

In the normally bustling historic Grand Place, a few bars and restaurants were open for business but they were struggling to find customers.

In downtown Brussels, the only real activity was deliverymen offloading crates for near-empty shops as builders hammered together stalls for a Christmas market meant to open on Friday.

"My grandson said we should up sticks and move to the south of the Yser river, just like in World War I (after the Germans invaded)," said Michel, a retired man from a Dutch-speaking suburb.

"We have to be careful, but life has to go on -- otherwise we're finished," said his wife Patricia.

The army and armed police will remain on the streets in coming days, the Belgian prime minister said, but schools and the metro system would reopen from Wednesday.

The European Union and NATO, which both have their headquarters in Brussels, said they would bolster security and urged non-essential staff to work from home.

The alert will be reviewed again on Monday.

Meanwhile 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 attacks, the fourth so far.

Mohammed Amri, 27, and Hamza Attou, 20, were charged on 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.

Abdeslam's brother Mohamed on Sunday told Belgian television he thought Salah had decided at the very last moment not to go through with his attack mission.

- 'Army of terrorists' -

France's Hollande said his country was "at war" with IS following the attacks, and on Monday planes based on the flagship Charles de Gaulle aircraft carrier launched their first attacks against the jihadist group in Iraq and Syria.

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 said he supported France's actions and added that "it's my firm conviction that Britain should do so too."

He will make his case to parliament on Thursday for Britain to expand its strikes as part of the US-led coalition into Syria.