Three suspected jihadists who smoothly passed France's border controls on their return from Syria, embarrassing the French government, have turned themselves into police.

Imad Djebali, Abdelouahed Baghdali, and Gael Maurize showed up at a police station in the village of Caylar in the southern Hérault department, a day after landing at Marseille airport.

"Our clients expressed from the very start, after landing, their intention to clarify their situation with police," said Pierre Le Bonjour, a lawyer for the trio. "We confirmed with them that was the right thing to do."

The three men returned to France via Turkey after almost six months spent in Syria fighting with Islamic State (IS) militants.

Their lawyers said they had decided to quit jihad after growing disillusioned with fighting for IS and crossed to Turkey, where they were arrested.

Ankara contacted French authorities to repatriate them and the trio was initially expected to arrive at Paris Orly airport on 23 September.

Djebali, Baghdali, and Maurize were known to anti-terrorism forces in France due to Mohamed Merah, a 23-year-old extremist who in 2012 went on a motorcycle shooting rampage, killing three soldiers, a rabbi, and three children at a Jewish school in Toulouse.

However, security forces waiting for them at Orly were left empty-handed as the three boarded a Marseille-bound flight instead, and border police in the Mediterranean city let them through without hassle.

Their lawyers said they were surprised they were not held.

"They came back to France of their own free will. The other reality is that they went through customs in France with their passports without being bothered in any way," said Pierre Dunac, who represents Djebali.

The government was accused of "ineptitude" by the political opposition, and the incident has cast doubt over the authorities' ability to ensure public safety from home-grown radicals returning to France after receiving training and combat experience in the Middle East.