Government authorities in Brussels extended the city's highest-level terror alert into next week as they maintained their warning Islamic State (IS) terrorists plan to attack highly populated areas such as public transport systems, cinema halls and shopping malls.

The Belgian authorities have charged a fourth person in connection with the Paris attacks, when IS suicide bombers and gunmen killed 130 people, as an international manhunt continued for mastermind Salah Abdeslam.

The French police on Monday said they had found a suspected explosives belt in a suburb near Paris where the 26-year-old was thought to have been on the night of the killings. The item was found in a dustbin in Montrouge, south of Paris, where a person close to the enquiry said telephone data placed Abdeslam on the night of 13 November.

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

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

The federal prosecutor's office announced that a man who was arrested during a police raid in Belgium late Sunday has been charged with involvement in the Paris attacks, the fourth so far.

On Monday, Mohammed Amri, 27, and Hamza Attou, 20, were charged on suspicion of helping Abdeslam escape to Brussels after the attacks, while a third unnamed person faces charges of helping him when he reached the city.

On Monday, French fighter jets based on the Charles de Gaulle aircraft carrier launched their first attacks against the jihadist group in Syria and Iraq.

French President Francois Hollande met British Prime Minister David Cameron in Paris in an effort to widen an international alliance against the extremist group.

Cameron will make his case to parliament on Thursday for the UK to expand its strikes as part of the US-led coalition into Syria in his latest bid to get lawmakers' approval for the move.