"We are still on level three, quite a high level of threat," said Benoit Ramacker, spokesman for the Crisis Center. Level three out of a possible four means the threat is considered serious, possible and probable.
The Derniere Heure tabloid reported 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.
The paper, which said it had obtained an alert message, said fighters traveling without passports were believed to be trying to reach Europe by boat via Turkey and Greece.
Ramacker said, "these kinds of potential targets are under protection anyway" and that "nothing has changed in terms of security."
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He declined to comment specifically on the reports, saying only that "there is a lot of information coming in." Brussels police said they had not seen any such warning from the anti-terror office.
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.