Police have been alerted to the possibility that small groups of extremists have left Syria for France and Belgium with plans to stage attacks, but have not raised threat levels, as authorities work to determine how credible the information is.
Belgian intelligence services sent a note to French counterparts about the possible groups, and it was sent to police and anti-terrorism forces across France on Tuesday, a French security official said on Wednesday.
French authorities remain "very cautious" about the information because they receive such notes routinely, the official said. Belgian officials are cautious too, as the information is believed to be from a single intelligence source and has not been confirmed elsewhere.
France and Belgium have already been under high alert since the Isis attack on Paris last year, that killed 130 people, and the March suicide attacks in Brussels that left 32 people dead. It has also threatened violence during the Euro 2016 football tournament taking place across France this month.
French and Belgian officials said the information does not change their governments' overall understanding of the threat.
In Belgium, security experts say the information "as such has no direct impact on the current threat level," according to Crisis Centre spokesman Peter Mertens.
The threat analysis centre said on Wednesday it is keeping Belgium's security alert status at its current level of three out of a possible four, which means the threat is considered serious, possible and probable.
Belgium's Derniere Heure tabloid reported on Wednesday that the country'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 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. Belgian authorities said such targets are already under surveillance.
Given the heightened security fears in France at the moment, it would not be uncommon for Belgian authorities to pass on information without immediately verifying its credibility, said a security official with knowledge of the Belgian intelligence information.
"The information regarding the possibility of a threat emanating from Syria, however, has not yet been verified," said the official. "It is also important to remember that credible intelligence shows there are hundreds of fighters already in Europe ready to strike. They are the real threat."
Since the recent attacks, there is pressure for European security officials to share most of the information they get, even if it turns out to be false, the official said. He said this is especially true given the attack in France, the Orlando shootings and security fears over the Euro 2016 tournament.
It is not the first report that fighters might have been dispatched to Europe since the 22 March suicide attacks on the Brussels airport and subway. On 19 April, Crisis Centre chief Paul Van Tigchelt said there were signals from Isis that fighters were sent to Europe, including Belgium.
A Brussels shopping centre, City 2, increased security after seeing the media reports, although it was not warned to by police.
"We reinforced our security and closed off some of the entries to make checks easier at the other entries," said spokeswoman Veronique Pirson.
The French official said it is unclear whether the Belgian note could have any link to an attack on Monday night by an Isis extremist who killed two French police officials.
The stabbing in a Paris suburb revived French concerns about the Isis threat. France's president and prime minister warned on Wednesday that the world faces a long war to defeat terrorism.
"I said we were at war, that this war will take a generation, that it will be long," Prime Minister Manuel Valls said on France Inter radio.
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