The refusal of the French government to take back Islamic State fighters from Syria could fuel a new jihadist recruitment drive in France, threatening public safety, a leading anti-terrorism investigator has told AFP.

David De Pas, coordinator of France's 12 anti-terrorism examining magistrates, said that it would be "better to know that these people are in the care of the judiciary" in France "than let them roam free".

Turkey's offensive against Kurdish militia in northeast Syria has sparked fears that some of the 12,000 jihadists, including thousands of foreigners, being held in Syrian Kurdish prisons could escape.

Officials in Paris say 60 to 70 French fighters are among those held, with around 200 adults, including jihadists' wives, being held in total, along with some 300 children.

France has refused to allow the adults return home, saying they must face local justice. So far Paris has only taken back a handful of children, mostly orphans.

This week, Foreign Minister Jean-Yves Le Drian travelled to Iraq to try convince Baghdad to take in and try French jihadists being held in northern Syria.

On Friday, in a rare interview, De Pas argued that instability in the region and the "porous nature" of the Syrian Kurdish prison camps risked triggering "uncontrolled migration of jihadists to Europe, with the risk of attacks by very ideological people".

- 'Infernal cycle' -

The Turkish offensive, which has detracted the Kurds' attention from fighting IS, could also facilitate the "re-emergence of battle-hardened, determined terrorist groups."

This in turn could spur the establishment of new jihadist networks to supply "French citizens drawn to these groups," he argued.

Warning of the risk of "a new vicious circle" of radicalised young French people travelling to Syria, De Pas called on the government to demonstrate "the political will to repatriate" the fighters.

Investigating magistrates are independent of the government, but it is extremely rare in France for them to publicly challenge policy.

Last month, IS chief Abu Bakr al-Baghdadi urged the group's followers to break jihadists and their family members out of prisons in Iraq and Syria.

According to Syrian Kurdish authorities, nearly 800 wives and children of foreign jihadists escaped from a camp for the displaced on the first weekend of the Turkish offensive.

At least three French women ended up in the hands of the Islamic State, according to their lawyer.

Making the case for the fighters to be brought home, De Pas pointed out that Paris had for years been successfully taking back and jailing jihadists expelled from Turkey.

"I understand that there may be nervousness but how we can we protect ourselves if we don't have our hands on them?" he asked.

The magistrate, who said he was speaking out on the issue because "I would feel responsible if I didn't say it", also warned against the temptation of transferring jihadists to Iraq to face justice there.

Once they had been tried and served their sentences in Iraq they would disappear under the radar, he said.

In France, by contrast, "if in 15, 20, 30 years these people still constitute a threat on leaving prison... they will remain under the watch of the intelligence services and the justice system."