Belgium's anti-terror authorities warned police that a group of fighters from the terror organization known as the Islamic State group may have left Syria 10 days ago with plans to attack France and Belgium, multiple Belgian newspapers reported Wednesday. The Belgian police have maintained the security alert at Level 3, refusing to raise it to the highest tier of Level 4, which indicates the immediate threat of attack.

Fighters from the terror group, also known as ISIS, were reportedly attempting to cross into Greece via boats and without using passports. Potential targets could include a U.S. fast-food chain, a Brussels mall and police personnel, according to a report in Belgian newspaper Dernière Heure.

"We are still on Level 3, quite a high level of threat," Benoit Ramacker, spokesman for Belgium’s threat analysis center, told Associated Press. "These kinds of potential targets are under protection anyway," he said, adding, "Nothing has changed in terms of security."

The warning comes just two days after a Paris police officer and his partner were stabbed to death at their home by a man who had pledged allegiance to ISIS. It also follows on the heels of Sunday morning's deadly shooting rampage at a gay nightclub in Orlando, Florida — the worst mass shooting in U.S. history, which has been linked to an ISIS supporter.

ISIS fighter
An Islamic State group fighter holds an ISIS flag and a weapon on a street in Mosul, Iraq, June 23, 2014. REUTERS/Stringer

Security has remained high throughout Belgium and France since the November terror attacks in Paris that left 131 dead and hundreds more wounded. A group of assailants who had pledged allegiance to ISIS stormed bars, restaurants, a concert hall and outside a stadium, using assault weapons and suicide vests in what would become the deadliest attack on French soil since World War II.

Belgium has maintained at least a Level 3 alert since the Paris attacks. Brussels issued a Level 4 alert after twin attacks on an airport and a city subway during rush hour in March left 32 dead and dozens more wounded.