The threat of a jihadist attack in France has reached a level "without precedent," and new attacks are inevitable, according to top counterterrorism officials.

"The threat is permanent," said one high-level official in the defense ministry, speaking on condition of anonymity.

Defense ministry specialists say the jihadists "use the best encryption and concealment techniques" and that security services are "playing catch-up".

"Every time we get our hands on a network, we see they are each using seven or eight SIM cards, changing them constantly. And the most cunning don't go near phones at all - they use messengers," the official said.

The biggest concerns relate to the estimated 200 individuals who have returned from training or fighting in areas held by the Islamic State group in Iraq and Syria.

"They have lost all inhibitions about violence," said another top counterterrorism official, also speaking on condition of anonymity.

Security services place them under the tightest possible surveillance, but resources are limited. The authorities are also painfully aware that militants may wait years before acting, as was the case with the Kouachi brothers who carried out the attack on Charlie Hebdo satirical magazine in January.

They had long-standing links to jihadist networks in Paris, and one had traveled to Yemen for training back in 2011. They had gradually fallen off the radar of security services after laying low for several years.

Officials also fear that competition between militant groups may drive them to evermore brutal acts.

"Al-Qaida needs to restore its prestige and will try to compete with IS with complex and major actions," said the official.

He highlighted the threat from an al-Qaida subgroup known as Khorasan, which is still thought to be planning a major airline attack.

One of Khorasan's key members is a French explosives expert, David Drugeon, who is thought to have survived an attempted assassination by a US drone strike last year.

Since the attacks in Paris in January that left 17 dead, France has been on the highest possible alert with thousands of police and troops deployed at sensitive sites, such as media headquarters and synagogues. But counterterrorism officials say this will do little to prevent an attack.