A fourth person has been charged with terror offences in Belgium in connection with the Paris attacks.

In a statement, the federal prosecutor said the suspect, who has not been identified, was charged with "participation in the activities of a terrorist group and a terrorist attack".

Fifteen people who were arrested at the same time in late-night raids on Sunday have been released, police said.

Two of five people detained on Monday were also released.

The government said that fugitive Salah Abdeslam, who is believed to have been the gunman who sprayed the Casa Nostra pizzeria with bullets, killing five people, was not among those detained in raids last night.

It is not known if he was captured in the latest raids.

At a news conference soon after midnight, officials said a total of 19 houses were searched in the city - and during the operations, police fired two shots at a vehicle in the Brussels district of Molenbeek.

One of those arrested was injured when his car tried to ram into police during an attempted getaway.

Eric Van der Sypt, a Belgian federal prosecutor, said no firearms or explosives were discovered.

Earlier, there were reports that Abdeslam was in a BMW heading towards the German border but authorities have now said that car was not linked to the ongoing operation.

Meanwhile, Belgium's Prime Minister Charles Michel has announced Brussels will remain on the highest terror threat level, which indicates that an attack is "imminent".

He said the rest of the country will remain on the second-highest level, to reflect an incident is "probable".

But he said the metro, schools and universities in Brussels will reopen from Wednesday.

With an alleged terrorist still at large, Mr Michel has expressed fears that individuals could "launch several attacks at the same time in multiple locations".

The army and police presence has been boosted in the capital to protect targets such as shops and public transport.

Belgium has been at the centre of investigations into the Paris attacks after it emerged that two of the suicide bombers lived in Molenbeek.

Belgian authorities have already charged Mohammed Amri, 27, and Hamza Attou, 20, who were arrested in Molenbeek last Monday on suspicion of helping Abdeslam escape to Brussels shortly after the massacre of 130 people.

A third person, who has not been named, was charged on Friday and is reported to have helped Abdeslam when he was dropped off.

A police source fears Abdeslam could be "trapped and desperate" in the city.

Eurostar is allowing anyone booked to travel to Brussels on Monday to postpone their trip.

Meanwhile, French police have found an object "resembling" an explosives belt, without a detonator, in the Paris suburb of Montrouge.

They say it is currently being analysed to determine if it could have been used in the Paris attacks.

Earlier French police issued a photograph of the third man involved in the suicide blasts at the Stade de France - with officials admitting they do not know who he is.

British PM David Cameron travelled to France today to meet French President Francois Hollande to discuss the response to the terror attacks.

He has announced an overhaul of British defence spending which he says is aimed at dealing with the threat from Islamic State.