The knifemen entered the church during morning mass near the northern city of Rouen, northwest of Paris, killing Father Jacques Hamel, an 85-year-old parish priest, and taking four other people hostage, one of whom was seriously wounded, Reuters reports.
It was the latest in a wave of attacks in Europe inspired by the Islamist militant group based in Iraq and Syria that is on the defensive against a U.S.-led military coalition in which France is a major partner.
“They forced him to his knees and he tried to defend himself and that’s when the drama began,” Sister Danielle, who escaped as the attackers slayed the priest, told RMC radio.
“They filmed themselves. It was like a sermon in Arabic around the altar,” the nun added.
Police shot the attackers dead as they emerged from the church in the Normandy town of Saint-Etienne-du-Rouvray.
Speaking at the scene of the attack, President Francois Hollande said Islamic State had declared war on France, which should “use all its means” within the law to fight the group.
The president called it a “dreadful terrorist attack” and told reporters the attackers had pledged allegiance to Islamic State, against which France has launched air strikes in Syria and Iraq. News agency Amaq, which is affiliated with Islamic State, said two of its “soldiers” were involved.
“The threat remains very high,” Hollande said.
The White House condemned the attack and commended the French police for their “quick and decisive response.”
The attack was the latest in a string of deadly assaults including the attack in Nice, southern France, on Bastille Day 12 days ago and four incidents in Germany, most recently a suicide bombing at a concert in Ansbach on Sunday.
Islamic State has called on its supporters to take action with any available weapons targeting countries it has been fighting.
The anti-terrorist unit of the Paris prosecutor’s office is investigating the attack. Police said one person had been arrested.