The Belgian crisis center has said the alert was sent to all police forces in the country.
However, there were no immediate plans to raise the security level to the maximum showing an imminent threat of an attack, the source added.
The terrorists in question “left Syria about a week and a half ago to reach Europe via Turkey and Greece by boat without passports,” Belgian newspaper DH reported, quoting the alert.
A source in the French Interior Ministry said Belgian authorities had sent the alert to their French counterparts, who were now reviewing the data. “We know the threat is very high,” the source said, as quoted by Reuters. “We’re reviewing all the elements (in the alert).”
“We know there are fighters who are coming back (to Europe),” French government spokesman Stephane Le Foll told a news conference Wednesday.
However, French authorities remain “very cautious” about the alert, as they receive such notes very often, AP quoted their source as saying.