A French security official says police have killed two attackers who used knives to seize hostages in a church near the Normandy city of Rouen on Tuesday.

A priest was killed on Tuesday when two armed assailants seized hostages at a church near the northern French city of Rouen, police said.

The official said the identities of the attackers and motive for the attack on Tuesday are unclear. Interior Minister Bernard Cazenevue is en route to the town of Saint-Etienne-en-Rouvray where the hostage-taking took place, according to the official, who was not authorized to be publicly named.

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 Daesh group.

Five people were inside the church in the Normandy town of Saint-Etienne-du-Rouvray when it came under attack, interior ministry spokesman Pierre Henry Brandet said.

Three of the hostages were freed unharmed, and another was fighting for their life.

The motivations for the hostage-taking were not yet clear, but the Paris prosecutor's office said the case was being handled by anti-terrorism prosecutors.

The incident comes as France remains on high alert nearly two weeks after Tunisian Mohamed Lahouaiej Bouhlel ploughed a truck into a crowd of people celebrating Bastille Day in the French Riviera city of Nice, killing 84 people and injuring over 300.

French President Francois Hollande, who is from Rouen, and Interior Minister Bernard Cazeneuve were on their way to the scene, their offices said.

An AFP journalist said the scene of the attack was crawling with emergency vehicles and police.

French Prime Minister Manuel Valls expressed his horror at what he called "a barbaric attack on a church".

"The whole of France and all Catholics are wounded. We will stand together," he wrote on Twitter.

Pope Francis voiced his "pain and horror" at the hostage-taking, according to the Vatican.

The archbishop of Rouen, Dominique Lebrun, urged all non-believers to join those of the church in "calling to God".

"The Catholic Church can take up no other weapons that prayer and fraternity between men," he said in a statement.

The Nice attack was the third major strike on France in 18 months and was claimed by Daesh.

Two attacks in Germany claimed by Daesh since then have also increased jitters in Europe.

After the attack in Nice, France extended a state of emergency giving police extra powers to carry out searches and place people under house arrest for another six months until January.

It was the fourth time the security measures have been extended since Islamic State jihadists struck Paris in November, killing 130 people at restaurants, a concert hall and the national stadium.