Intelligence suggests that small groups of extremists are planning attacks in France and Belgium after leaving Syria.
Belgian intelligence services informed their French counterparts, who have now advised police across France.
A French security official said the authorities remained "very cautious" about the information because they received such communications routinely.
The official added that it was unclear whether there was any link to the murder of a senior policeman and his partner on Monday by a man who pledged allegiance to Islamic State.
The government's overall understanding of the threat had not changed, the official said.
French authorities are also on high alert over concerns that extremists could be planning to target the Euro 2016 football tournament.
A Belgian newspaper, Derniere Heure, citing Belgium's anti-terror office, reported that fighters with access to weapons may have left Syria about 10 days ago, bound for Belgium and France.
The paper said it had obtained an alert, saying that fighters travelling without passports were believed to be trying to reach Europe by boat via Turkey and Greece.
Possible targets were said to be a Brussels shopping centre, an American fast-food chain and police.
Belgium's security threat analysis centre said it would not be changing its advice.
We are still on level three, quite a high level of threat," said Crisis Centre spokesman Benoit Ramacker.
Level three - of four - means the threat is considered serious, possible and probable.
Belgium has been on level three or above since the Paris attacks in November.
Mr Ramacker declined to comment on the latest intelligence, saying "there is a lot of information coming in."
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