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

A French security official said Wednesday that Belgian intelligence services alarmed French counterparts through a note about the possible groups that was sent to police across France on Tuesday,

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 European Championship soccer 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.

Belgium’s Derniere Heure tabloid reported Wednesday that Belgium’s anti-terror office has warned police that fighters with access to weapons could have left Syria about 10 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 said fighters traveling without passports were believed to be trying to reach Europe by boat via Turkey and Greece.

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 mobilized.

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 Center chief Paul Van Tigchelt said there were signals from Islamic State that fighters were sent to Europe, including Belgium.