Brussels (AFP) - A vast police operation was under way in Brussels on Wednesday after a Kalashnikov-wielding assailant was killed and four officers wounded during an anti-terror search linked to the Paris attacks.

Federal prosecutors were to hold a news conference at 0930 GMT amid widespread media reports that two suspects were either arrested or still on the run, possibly brothers with links to terrorism.

"The investigation continues and the danger remains," Belgian Prime Minister Charles Michel told the broadcaster RTL, though he refused to provide any further details concerning an ongoing investigation.

Michel said operations had continued throughout the night. He did not rule out that the terror alert in Brussels could be raised to the highest level after a meeting of the national security council later in the day.

- Gunfire in the suburbs -

A team of Belgian and French police were taken by surprise as they carried out a routine search Tuesday on a property in the quiet suburb of Forest, or Vorst by its Dutch name.

The police intervention sparked a series of gun battles that wounded four officers and left children trapped in nearby schools.

Michel thanked residents for their "composure" as bursts of gunfire erupted in the streets and dozens of heavily-armed police with balaclavas and sub-machineguns patrolled the area.

The mayor of Forest, Marc-Jean Ghyssels, told RTBF radio that all schools and nurseries in his jurisdiction were open on Wednesday and that life was back to normal.

"There wasn't a direct threat to the public, it's a search that turned badly," he said.

The dead person had not been identified but prosecutors said it was not Salah Abdeslam, a key suspect in the November 13 attacks in Paris claimed by the Islamic State jihadist group, which killed 130 people.

A French police source said the operation was focused on the associates of one, or several, of 11 people who have been charged in Belgium in connection with the Paris attacks.

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"In this operation, one or several people opened fire on the police as they came through the door" of the property in an initial search, the federal prosecutor's office said in a statement.

"One suspect armed with a Kalashnikov-type assault rifle was killed in an assault" by police about three hours later, it said, adding that officers were "actively pursuing investigations, day and night".

A French policewoman, who Justice Minister Koen Geens said had been helping in a joint investigation between French and Belgian police, was among three officers wounded in the initial assault. A fourth was hurt in an exchange of fire.

Security forces launched a huge operation after the gun battle broke out, cordoning off the area with vehicles while a police helicopter hovered overhead, AFP reporters at the scene said.

- Brussels lockdown -

As the scene unfolded, children and teachers in two schools and two nurseries nearby were asked to remain indoors. They were eventually let out one-by-one, accompanied by armed police.

Police later started letting people back into their homes and 50 people who had been trapped in an Aldi supermarket were also allowed out.

By early Tuesday calm had largely returned to the area and most locals had been allowed to return to their home. Access to the scene of the shootout was also reopened.

The incident took place across the street from an Audi auto factory and the train lines leading to the Eurostar cross-channel terminal at the Gare du Midi railway station.

Soldiers are still on guard at key areas including train stations and EU institutions.

Two weeks after the Paris attacks, Brussels was put on five days of lockdown with authorities warning of an imminent threat of violence as the manhunt for Abdeslam continued.

Abdeslam, 26, who is believed to have played a key role in organising the Paris attacks, fled across the border to Belgium hours after the carnage and is now one of the most wanted men in Europe.

He was reportedly holed up for three weeks after the Paris attacks in an apartment in the Schaerbeek district in north Brussels, where police found a fingerprint, traces of explosives and possible suicide belts.