Muslims have attended Catholic Masses in churches and cathedrals across France in a gesture of solidarity after the gruesome killing of a priest in Normandy.

Muslims and Christians have prayed together at a mosque in the French town where an elderly priest was killed this week, with one imam calling the extremists non-Muslims who are "not part of civilisation" or "humanity".

French officials have identified the second man involved in the murder of a priest in a Normandy church during morning Mass.

British Prime Minister Theresa May has vowed that Islamist terrorists “will not prevail” in the wake of Tuesday’s murder of a Catholic priest in northern France.

Catholic Archbishop of Dublin, Diarmuid Martin, is concerned that the fatal stabbing of Fr Jacques Hamel, in France, may spur copycat killings.

Geneva police have increased a deployment at the city's airport and are conducting a sweep of identity papers and other checks after receiving unspecified information in connection with an investigation in France.

Ireland’s Catholic primate Archbishop Eamon Martin joined in the voicing of global shock as he offered prayers to those affected by the terrorist murder of an 86-year-old priest at his church in France.

One of the men who attacked a Normandy church, killing an 85-year-old priest, twice used relatives' ID cards in attempts to reach Syria, France's anti-terrorism prosecutor has said.

When militants loyal to Islamic State seek to inflict pain on Europe, France is their preferred target, a grim reality borne out yet again with today's killing of a Catholic priest.

A nun who escaped said she saw the attackers video themselves.

Latest: One person has been arrested in France after an attack on a church in which an elderly priest was killed.

A man once flagged for ties to Islamic radicals has rammed a vehicle into a gas factory in France, triggering an explosion that injured two people before the severed head of his employer was found hanging at the factory entrance.