Two jihadists murdered an 85-year-old priest celebrating Mass in a church near Normandy Tuesday — one of them forcing him to kneel near the altar before slitting his throat as the other captured the gruesome act of terror on video, French officials said.

A nun who escaped said she saw the attackers give a sermon in Arabic at the altar as they carried out their barbaric attack.

The two had fake explosives and used some nuns as human shields, a prosecutors said, adding that the attackers claimed allegiance to ISIS, and cried out, “Allahu akbar,” during the attack.

The Islamic State quickly claimed responsibility for the attack, in which the assailants stormed the church in the town of Saint-Etienne-du-Rouvray and took the Rev. Jacques Hamel, two nuns and two worshipers hostage.

“They forced him to his knees and obviously he wanted to defend himself and that’s when the drama began,” said one nun, who identified herself as Sister Danielle, The Guardian reported. “They were filming themselves preaching in Arabic in front of the altar. It was a horror.”

Police killed the two attackers as they came out of the church. One other person was arrested in connection with the attack, the 16-year-old younger brother of someone wanted by police for trying to go to Syria or Iraq in 2015.

One of the attackers was identified as 19-year-old Adel Kermiche, a Saint-Etienne-du-Rouvray resident who had tried to travel to fight for ISIS in Syria in 2015, BFM-TV reported. He was sent back by Turkish authorities and jailed in France.

He was released in March, and allowed to live with his parents near the church while wearing an electronic bracelet that monitored his movement — allowing him to be out between 8:30 a.m. and 12:30 p.m. The church was attacked at 9:40 a.m.

The church was one of several houses of worship that appeared on a hit list discovered on an ISIS suspect in April 2015, sources told the Express of the UK.

French President Francois Hollande, visiting the scene of the “ignoble terrorist attack,” said ISIS had declared war on France, which was already reeling from recent massacres at the hands of ruthless jihadists.

“We are confronted with a group, Daesh, which has declared war on us,” he said, using an alternative name for ISIS. “We have to wage war, by every means, (but through) upholding the law, which is because we are a democracy.”

ISIS claimed responsibility shortly after the attack.

“The perpetrators of the Normandy church attack are soldiers of the Islamic State who carried out the attack in response to calls to target countries of the Crusader coalition,” the group said on its Amaaq news agency.

Pope Francis expressed his “pain and horror,” according to the Vatican.

The attack comes amid a spate of terror strikes in France, including Tunisian Mohamed Lahouaiej Bouhlel’s truck rampage in Nice on July 14, when he plowed into a crowd of Bastille Day revelers – killing 84 and wounded over 300.

After the attack in Nice, the country extended a state of emergency giving authorities extra powers to carry out searches and place suspects under house arrest for six additional months until January.

France has been concerned about church attacks ever since Algerian student Sid Ahmed Ghlam, 24, was found with a hit list when he was arrested in Paris in April 2015 for allegedly killing a woman.

Authorities found documents about ISIS in his apartment and believe he had been in touch with a jihadist in Syria about an attack on a church.

French authorities said Ghlam was sent by the Belgian Abdelhamid Abaaoud to attack a church in Villejuif, just outside Paris.

A cell directed by Abaaoud carried out the Nov. 13 attacks in Paris that left 130 people dead and the March 22 attacks in Brussels that killed 32.

Ghlam is is being held on terrorism-related charges.

With Post Wires