A man who rammed a car into a police van in Paris stored a cache of weapons at his home and held a gun permit despite being on a secret service list of people linked to radical Islam, police sources and French officials have said.

A source close to the investigation said the 31-year-old assailant had sworn allegiance to the self-proclaimed Islamic State group in a letter to his brother-in-law.

Investigators were compiling an inventory of the arms and equipment found in his home.

The man, who died in the attack, was also carrying in his car an assault rifle, two pistols, ammunition and two large gas canisters when he rammed a police convoy yesterday.

Prime Minister Edouard Philippe said the individual first received a permit to possess a gun before he was flagged to intelligence agencies as a potential militant threat.

At the time there was no reason to deny him the permit, Mr Philippe said.

He said it was "quite possible" the licence was active at the time the attacker was on a security watchlist.

Three sources close to the investigation confirmed it was.

"Nobody can be happy, and certainly not me, that someone who has been flagged to security agencies can continue to benefit from such an authorisation," Mr Philippe told BFM TV.

The man was placed on France's so called 'Fiche S' watchlist after he was found to belong to a radical Islamist movement, two police sources said.

Individuals on the list are placed under surveillance though the intensity of that surveillance varies depending on the perceived threat the individual poses.

Interpol issued a search notice for the attacker in 2013 on the request of Tunisia but that was no longer active, one source said.

Police arrested four of the man’s close relatives in a raid south of Paris late yesterday, a police source said. They included his father and brother.