A Catholic priest was forced to kneel by two men armed with knives before they killed him by slitting his throat.

A nun who witnessed the deadly attack said the killers of 86-year-old Father Jacques Hamel also filmed the murder at the parish church in Normandy and gave a "sermon around the altar in Arabic".

An 86-year-old worshipper is also in a serious condition in hospital after suffering severe knife wounds during the hour-long siege, which started after the knifemen stormed the building in Saint-Etienne-du-Rouvray, near Rouen, during morning mass.

They took Father Hamel, three nuns and two worshippers hostage.

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Image: Father Jacques Hamel was killed in the church by the hostage-takers

The alarm was raised after one of the nuns escaped.

The murderous pair, both carrying fake bombs and one a handgun, were shot dead by police as they charged out of the church shouting "Allahu Akbar" (God is greatest), using the nuns as human shields.

One of the attackers has been identified as 19-year-old Adel Kermiche, who was from the town where the atrocity happened.

Known to the security services, he had twice attempted to travel to Syria and was under house arrest with an electronic tag, but this was turned off for a few hours each morning to allow him to leave home.

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Describing the murder, the nun, identified as Sister Danielle, told French TV: "They forced him to his knees. He wanted to defend himself. And that's when the tragedy happened.

"They recorded themselves. They did a sort of sermon around the altar in Arabic. It's a horror."

She said of her dead colleague: "He was an extraordinary priest. He was a wonderful, kind man."

Image: The body of one of the two men who stormed the church

Islamic State claimed two of its "soldiers" carried out the deadly assault.

Visiting the scene of the atrocity, French President Francois Hollande described it as a "vile terrorist attack" and vowed to fight the IS extremists "using all means possible".

Anti-terror chiefs have been appointed to take charge of the investigation and have arrested one person in connection with the attack.

French media have also reported that the church targeted was one of several Catholic sites on the hit-list of a 24-year-old Algerian student.

Image: The church where the attack took place

Sid Ahled Ghlam was arrested last year on suspicion of murdering a mother-of-one during a botched attempt to attack a church in Villejuif.

He was sent by Belgian Abdelhamid Abaaoud, who directed a cell which carried out the 13 November attacks in Paris during which 130 people were killed and the 22 March attacks in Brussels that killed 32 people.

Pope Francis also expressed his "pain and horror" and said he was appalled by the "barbaric killing".

The attack comes almost two weeks after the Bastille Day massacre in Nice which left 84 people dead after Tunisian Mohamed Lahouaiej Bouhlel drove a truck into a crowd celebrating the national holiday on 14 July.