One hostage and two hostage-takers were killed in northern France after an incident at a church near the Normandy city of Rouen on Tuesday.
The attackers killed one hostage by slitting their throat before being neutralised by police, a security official said.
The identities of the attackers and motive for the attack are unclear, according to the official, who was not authorized to be publicly named.
French President Francois Hollande and Interior Minister Bernard Cazeneuve are en route to the town of Saint-Etienne-en-Rouvray where the hostage-taking took place, Interior Ministry spokesman Pierre-Henry Brandet said on France-Info radio.
Anti-terrorism investigators have been summoned in the case, Brandet said.
The incident comes as France is under high alert after an attack in Nice that killed 84 people and a string of deadly attacks last year claimed by the Islamic State group. France is also under a state of emergency and has extra police presence in the wake of the July 14 Nice attack.