The French government has moved to boost security following the latest terror attack in the country.

19-year-old Adel Kermiche has been named as one of the men responsible for the brutal killing of Fr Jacques Hamel in Saint-Etienne-du-Rouvray.

Interior Minister Bernard Cazeneuve said France would bolster the operational reserve of its police force after 2,500 people asked to join up in the days after 84 people died in an IS-inspired attack on Bastille Day.

He also said more of the country's 10,000-strong Operation Sentinel anti-terror forces would be deployed to areas outside Paris following the lorry attack in Nice and the killing in Normandy.

He spoke after former president Nicolas Sarkozy accused Mr Hollande of being "out of touch" and called for detention or electronic tagging of all suspected Islamist militants - even if they have committed no offence.

But Mr Cazeneuve said: "We can't step back from the rule of law to protect the rule of law.

"If we abandon constitutional principles to protect that which we hold most dear - our liberty - we will be giving a victory to the terrorists."

Security forces in France are under scrutiny after it emerged one of the men behind the Normandy murder was under house arrest and wearing a tag, having twice tried to travel to Syria.

Adel Kermiche's tag was turned off for a few hours each morning to allow him to leave home - and it was in this time that Kermiche and another attacker slit the 86-year-old's throat.

Both men were shot dead by police. Details of the second man have yet to be released.

Series of attacks

Police officers prevent the access to the church where an hostage taking left a priest dead the day before in Saint-Etienne-du-Rouvray, Normandy. Image: Francois Mori / AP/Press Association Images 

The killing came just two weeks after the Bastille Day attack and is the latest of more than a dozen attacks attributed to Islamic extremists in France over the past two years.

Mr Hollande met with France's main religious leaders on Wednesday morning before attending a defence council and a cabinet meeting.

After the meeting, Paris Archbishop Cardinal Andre Vingt-Trois urged Catholics not to "enter the game" of IS that "wants to set children of the same family in opposition to each other". 

French authorities are again trying to establish whether the Normandy attackers were part of a network after Islamic State claimed its "soldiers" were responsible.

Paris prosecutor Francois Molins said Kermiche, 19, first came to the attention of anti-terror officials when a family member alerted them that he was missing in March 2015.

German officials arrested him and found he was using his brother's identity while trying to travel to Syria.

He was released under judicial supervision, but in May fled to Turkey where he was again arrested and returned to France. He was then held in custody until March this year.

A neighbour of Kermiche in Saint-Etienne-du-Rouvray, who gave his name only as Redwan, said he had known who was responsible as soon as he heard about the attack.

"I was told that an attack occurred and I knew it was him, I was sure," he said.

"He said to us 'Yes, I tried (to go to Syria).' Then we tried to bring him to his senses, every time we did it and every time he was bringing in a verse from the Koran, he was inventing things."

An 86-year-old worshipper is also in a serious condition in hospital after suffering knife wounds during the hour-long siege in which Fr Hamel, three nuns and two churchgoers were taken hostage.

The alarm was raised after one of the nuns escaped.

The attackers both carried fake bombs and one had a handgun.

They charged out of the church shouting "Allahu Akbar" (God is greatest), using the nuns as human shields, before being shot dead.

A nun identified as Sister Danielle, who witnessed the killing, 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."