Belgian intelligence services sent a note to French counterparts about the possible groups, and it was sent to police across France. 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 government’s overall understanding of the threat.
France is already under high alert because IS extremists targeted Paris last year and have threatened violence during the Euro 2016 football tournament.
Belgium’s Derniere Heure tabloid reported that Belgium’s anti-terror office has warned police that fighters with access to weapons could have left Syria about 11 days ago bound for Belgium and France. It was the latest of several recent reports that fighters from Syria could pose an imminent threat.
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 centre, an American fast-food chain and police could be among their targets.
Belgium’s security threat analysis centre said 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.
Mr 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 that “there is a lot of information coming in”.
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 is 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 Islamic State that fighters were sent to Europe.