The man suspected of decapitating his boss in an attack on a gas factory in France has confessed to the grisly crime, sources say.

Yassin Salhi, 35, “has also given details about the circumstances” surrounding the killing, according to identities close to the investigation, who said he would be transferred to Paris for further questioning later on Sunday by anti-terrorist police.

Salhi’s confession came after it emerged the married father of three sent a gruesome selfie photo including the severed head to a WhatsApp number in Canada.

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Investigators have warned, however, that it could be a relay number and the intended recipient could be anywhere in the world.

After several hours of silence, Salhi has begun to open up to investigators about the assault, which came six months after 17 people were killed in Islamist attacks in Paris that began with the Charlie Hebdo massacre.

Herve Cornara was found beheaded and pinned to the fence of a chemical factory. Photo: AAP

On Friday morning, Salhi rammed his van into the US-owned Air Products factory near France’s second city of Lyon in what President Francois Hollande said was a “terrorist” attack designed to blow up the whole building.

He was overpowered by a firefighter as he was trying to prise open a bottle of acetone in an apparent suicidal bid to destroy the factory.

Police then made the grisly discovery of the severed head of Salhi’s boss, 54-year-old Herve Cornara, lashed to the gates of the factory near two flags on which were written the Muslim profession of faith.

Prime Minister Manuel Valls repeated that the world was engaged in a “war against terrorism”.

“We cannot lose this war because it’s fundamentally a war of civilisation,” Mr Valls told the iTELE rolling news channel.

“It’s our society, our civilisation that we are defending.”

France was facing “a major terrorist threat” that needed to be fought “over the long term”, warned the prime minister.

It was not a question of whether there would be another attack, but “when” and “where”, stressed Mr Valls.

Friday’s attack came on a day of bloodshed on three continents that saw 38 people mown down on a Tunisian beach and 26 killed in a suicide attack in Kuwait.

The Islamic State extremist group has claimed responsibility for those two attacks, but no group has said it carried out the French operation.

A French flag and a white rose on the fence of a factory where a man was beheaded. Photo: AAP

Sources close to the investigation said Salhi was radicalised more than a decade ago after contact with Muslim convert Frederic Jean Salvi – known as “Ali” – who is suspected of preparing attacks in Indonesia with al-Qaeda militants.

An autopsy on Salhi’s victim has proved inconclusive, with experts unable to determine whether he was killed before being beheaded or decapitated alive.

The French investigation is naturally focusing on Syria, where hundreds of people from France have gone to wage jihad, officials say.

Anti-terrorist authorities have identified 473 people who have left France to fight in Iraq or Syria.

Mr Valls said 1800 people in France were “linked” in some way to the jihadist cause.

Interior Minister Bernard Cazeneuve said on Friday that Salhi had been investigated for links to radical Salafists in Lyon, but was not known to have participated in terrorist activities and did not have a criminal record.

-AAP