Belgian authorities have charged a fourth suspect in connection with the Paris attacks,while releasing 17 other people, the federal prosecutor’s office says.

The prosecutor’s office announced a man who was arrested during a large police operation in Belgium late on Sunday had been charged with involvement in the Paris atrocities.

“He is charged with participating in activities of a terrorist group and with a terrorist attack [Paris],”the office said in a statement.

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Belgian police arrested five more people in a second series of anti-terrorism raids as the capital, Brussels, was locked down for a third day under a state of maximum alert.

The arrests had brought to 21 the total number of people detained since police launched Sunday’ security operation.

No details on the suspects arrested was given. Seventeen of those arrested are reported to have now been released after being questioned.

Salah Abdeslam is believed to be on the run after the attacks in Paris.

About 26,000 euros ($38,380) in cash was seized during one of the searches and other items seized by police were being examined, the statement said.

Two of those charge – Mohammed Amri, 27, and Hamza Attou, 20 – were charged last Monday on suspicion of helping key suspect Salah Abdeslam escape to Brussels after the attacks in the French capital this month, which killed 130 people.

A third, unnamed person faces charges of aiding him when he reached the city.

Abdeslam is believed to remain on the run after officials said he was not among those arrested on Sunday night.

His mobile phone has been detected in the 18th district in the north of Paris and then later in Chatillon in the south, the source said.

An incident in which a BMW car rushed off when pulled over by police for a routine check near Liege on Sunday – which some Belgian media said involved Abdeslam –  had “no link at all with the ongoing operation” and the vehicle had been identified, they added.

Meanwhile, Belgian soldiers continue to patrol the streets of Brussels, while authorities are still warning of possible imminent attacks like those in Paris.

Belgian prime minister Charles Michel said the city would be kept under the maximum, level-four terror alert for another week, but that schools and the metro system would reopen from Wednesday.

‘Explosive belt’ found in rubbish bin

Meanwhile, material believed to be an explosive belt has been found in a public rubbish bin in the town of Montrouge south of Paris, a source close to the investigation said, 10 days after attacks in the capital left 130 people dead.

The object was found in the bin on Monday afternoon (local time), a police source said, confirming information reported by France Info radio.

The belt, which was found by garbage men, is being analysed “to confirm whether it is explosive”.

The source said telephone data placed Abdeslam in the Montrouge area on the night of the attacks.

Cameron offers Hollande support

David Cameron stands united with France. Photo: AAP

British Prime Minister David Cameron has offered French president Francois Hollande full support in the fight against the Islamic State (IS) group.

“I firmly support the action president Hollande has taken to strike ISIL in Syria,” Mr Cameron said after talks in Paris on Monday, using another acronym for IS.

“It’s my firm conviction that Britain should do so too,” he said, offering France the use of a British airbase in Cyprus for strikes against IS in Syria and Iraq.

The two leaders earlier laid a wreath at the Bataclan concert venue in Paris where 90 people died on November 13.

People going to work amid heavy police presence

Armed police and troops patrolled the near-deserted streets of Brussels all weekend after the government raised the terror alert to the highest level of four in the city.

Police patrol the Eiffel Tower after the terror attacks.

They were out in force again on Monday, but people were trying to go about their daily lives as normally as possible in the circumstances.

Traffic was relatively light in the centre of the city and many people opted to cycle to work.

In the normally bustling historic Grande Place, a few bars and restaurants were open for business but it was hard going to get customers.

The European Union and NATO both said they would bolster security and urged non-essential staff to work from home.

The rest of the country, including Brussels airport, remains on security alert level three, meaning an attack is considered possible and the threat credible.

Belgian prime minister Charles Michel announced on Monday evening (local time) schools and the metro were expected to reopen on Wednesday.