PARIS (AFP) - At least two of the gunmen who unleashed terror on France are believed to have been radicalised in prison, a fertile ground for extremism that authorities are struggling to contain.

Both Mohamed Merah, the Al-Qaeda militant who shot dead seven people in a series of 2012 attacks, and Mehdi Nemmouche, last year's Brussels Jewish museum killer, were radicalised in jail.

And in the latest attacks, Cherif Kouachi, one of the brothers who massacred 12 people in an attack on the Charlie Hebdo magazine, came under the influence of Djamel Beghal, a known figure of French radical Islamism, when serving time in the Fleury-Merogis prison.

Amedy Coulibaly, the man who killed a policewoman and four shoppers, also came under Beghal's influence and met Kouachi in the same jail, Europe's largest.

"We have a real problem with our prison policy," said Martin Pradel, a lawyer who has defended jihadists.

"Promiscuity, idleness and a lack of psychological support is all fertile ground for radical practices to bloom." Guy Guenoun, another lawyer, said a top criminal had warned him of the dangers of radical Islam in prison before the Merah affair.

- The justice ministry, while acknowledging the problem, says it is not quite as dire as believed.

"Of the 152 people currently jailed in France (out of 67,000 prisoners) as part of terrorist cases, just 16 percent of them had already served time in prison, 84 percent therefore radicalised elsewhere," said ministry spokesman Pierre Rance.

And among the 152, "around 60 have been identified as preachers engaged in proselytism among other Muslim detainees," he added.

To fight against their influence, the Fresnes prison just outside Paris - which houses around 15 of them - recently decided to separate them from the others by grouping them together in a section of the building.

The move is a first in France and a test for the prison administration, but critics say it is unlikely to work.

"Creating small Guantanamos is not the solution," said Pradel, who added this would only reinforce the detainees' radicalisation.

Ahmed El Houmass, a member of the CGT Penitentiary union, added it would create "zones of lawlessness."

The justice ministry also aims to fight against Islamist proselytism by increasing the number of mainstream Muslim chaplains in prisons.

One chaplain said that in the absence of legitimate preachers, detainees seeking spiritual guidance will seek out other prisoners who may mislead them.

"As by nature people hate emptiness and there are not enough chaplains, detainees who have a spiritual need go to inmates who practice (religion) ...," Yaniss Warrach, a chaplain at the Alencon-Conde prison in northwestern France, recently told AFP.

The ministry says it has created 32 such posts since 2012.

"There are now 183 in French prisons," it said, adding hundreds of thousands of euros have been invested.

Another challenge for prison administrations is identifying those detainees who could be prone to extremism, as the most radical are not necessarily those who display their religion ostentatiously.

"On the contrary, they're not noticeable," said an official at the justice ministry, who pointed out that Coulibaly posed no problem in prison.

As such, the ministry has launched an action plan that aims to have prison officials, specialist researchers and organisations such as the Arab World Institute collaborate to find ways to detect those easily influenced or to help de-radicalise them.