Police and anti-terrorism investigators have been alerted to the possibility that small groups of ISIS extremists have left Syria for France and Belgium with plans to stage attacks.

Belgian intelligence services sent a note to French counterparts about the warning and it was sent to police across France on Tuesday, a French security official said today.

Belgium's Derniere Heure tabloid reported that Belgium's anti-terror office warned police that fighters with access to weapons may have left Syria about 10 days ago for Belgium and France.

The paper, which said it had obtained an alert message, said fighters travelling without passports were believed to be trying to reach Europe by boat via Turkey and Greece.

A Brussels shopping mall, an American fast-food chain and police could be among their targets.

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Belgian soldiers patrol in the City2 shopping centre in central Brussels as it emerged that a terror alert had been sent to police that ISIS fighters were heading to Europe from Syria to target a Brussels shopping mall

Belgium's security threat analysis centre said it is keeping the country's security alert status at its current level of three which means the threat was serious, possible and probable

It was the latest of several recent reports that fighters from Syria could pose an imminent threat.

French authorities remain 'very cautious' about the information because they receive such notes routinely, the official said.

The official said the information does not change the French government's overall understanding of the threat.

France is already under high alert because ISIS extremists targeted Paris last year and have threatened violence during the Euro 2016 football tournament taking place at the moment.

The official, who spoke on condition of anonymity to be able to discuss sensitive security information, had no information about specific targets or how many extremists could be en route.

Security forces patrol in and around the gare de Marseille-Saint Charles train station for Euro 2016.

On alert: Flowers are left near the house where French police officers Jean-Baptiste Salvaing and partner Jessica Schneider were murdered by ISIS assailant Larossi Abballa, in Magnanville, near Paris

Belgium's security threat analysis centre said Wednesday it is keeping the country's security alert status at its current level.

'We are still on level three, quite a high level of threat,' said Benoit Ramacker, spokesman for the Crisis Centre.

Level three out of a possible four means the threat is considered serious, possible and probable.

Ramacker said: 'These kinds of potential targets are under protection anyway' and that 'nothing has changed in terms of security.'

He declined to comment specifically on the reports, saying only that 'there is a lot of information coming in.'

Pledged allegiance to ISIS: The stabbing of a married police couple in a Paris suburb earlier this week by 25-year-old Larossi Abballa (pictured) has revived French concerns about the ISIS threat

Jean-Baptiste Salvaing (left) was killed first before the killer turned on his partner Jessica Schneider (right)

Belgium has been on level three or above since November in the wake of the massacres in Paris that killed 130 people, with extra police and military mobilised.

It's not the first report that fighters might have been dispatched to Europe since the March 22 suicide attacks on the Brussels airport and subway that killed 32.

On April 19, Crisis Centre chief Paul Van Tigchelt said there were signals from ISIS that fighters were sent to Europe, including Belgium.

The French official said it is unclear whether the Belgian note could have any link to an attack Monday night by an ISIS extremist who killed two French police officials.

The stabbing in a Paris suburb earlier this week revived French concerns about the ISIS threat.

France's president and prime minister warned on Wednesday that the world faces a long war to defeat terrorism.