French police were on Tuesday hunting for a group of gunmen who killed two prison officers in an attack at a motorway toll that freed a convict linked to gangland drug killings.

President Emmanuel Macron vowed that everything would be done to find those behind the attack, with the inmate also still at large as well as those who helped him escape.

Two prison officers were killed in the attack and three others were wounded, Paris prosecutor Laure Beccuau said.

Another was fighting for his life in hospital and two more were receiving critical care, she said. 

The incident took place late on Tuesday morning at a road toll in Incarville in the Eure region of northern France.

The inmate was being transported after being questioned by a judge in the regional centre of Rouen in Normandy back to his prison in the town of Evreux.

The prosecutor said the prison van was rammed head-on by a stolen Peugeot vehicle as it went through the toll crossing.

But the prison vehicle was also followed by an Audi from which gunmen emerged who shot at both vehicles in the prison convoy.

"Everything is being done to find the perpetrators of this crime," Macron wrote on X.

"We will be uncompromising," he added, describing the attack as a "shock".

French television channels broadcast footage of the attack taken by surveillance cameras at the toll, showing the vehicle colliding head on with the prison van.

In the video, the doors of the car are flung open and several gunmen dressed in black emerge. A firefight ensues and one individual appears to be guided away from the van by the gunmen. 

A vehicle believed to have been used by the attackers was later found as a burned-out wreck at a different location. 

- 'They will be punished' -

"These are people for whom life counts for nothing. They will be arrested, they will be judged and they will be punished according to the crime they committed," said Justice Minister Eric Dupond-Moretti.

Hundreds of police and gendarmes have been mobilised to take part in the search, according to Interior Minister Gerald Darmanin.

The prison officers who died, both men, were the first to be killed in the line of duty since 1992, he added.

One of them was married and had two children while the other "left a wife who is five months pregnant", he said.

Prison officer unions announced a day of minimum service on Wednesday and asked for urgent measures to improve the safety of staff.

Dupond-Moretti said he would meet union representatives Wednesday.

The prison convoy had no police escort, according to a source close to the case.

A unit of the GIGN elite police force has been dispatched to try and apprehend the suspects.

- 'Inexcusable violence' -

Prosecutor Beccuau named the inmate as Mohamed Amra, born in 1994, saying that last week he had been convicted of aggravated robbery and charged in a case of abduction leading to death.

The case has been handed to prosecutors from France's office for the fight against organised crime, known by their acronym JUNALCO.

But a source close to the case said that Amra was suspected of involvement in drug trafficking and of ordering gangland killings.

Another source said he is suspected of being at the head of a network. Some media said he had the nickname "La Mouche" (the fly).

His lawyer Hugues Vigier said Amra had already made an escape attempt at the weekend by sawing the bars of his cell and said he was shocked by the "inexcusable" and "insane" violence.

"This does not correspond to the impression that I had of him," the lawyer told BFMTV.

The incident came on the same day as the French Senate published a damning report warning that government measures had been unable to prevent the flourishing of the narcotics industry in France.

Committee chairman Jerome Durain said France was "not yet a narco-state" but drug trafficking nonetheless constituted "a direct threat to the national interest" and the government's anti-drugs measures were "not up to the challenge."

Law and order is a major issue in French politics ahead of next month's European elections and the prison van ambush sparked fierce reactions from politicians, especially the far right.

"It is real savagery that hits France every day," said Jordan Bardella, the top candidate for the far-right National Rally (RN) which is leading opinion polls for the elections.