Dozens of French teenagers, including a young Jewish girl, have fled the country to join Islamic State militants fighting in Syria and Iraq, French intelligence has revealed.

At least 100 girls and young women from France have left to join terrorists in Syria in recent weeks, up from just a handful 18 months ago when the trip was not even on Europe's security radar.

The departures are less the whims of adolescents and more the conclusions of months of work by organised radical recruitment networks that specifically target young people in search of an identity.

These mostly online networks recruit girls to serve as wives, babysitters and housekeepers for jihadists, with the aim of planting multi-generational roots for a long-lasting Islamic caliphate.

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Lost: Kamel Ali Mehenni and his wife Severine hold pictures of their daughter Sahra at their home in Lezignan Corbieres, France. The image on the left is a frame grab taken from a CCTV camera showing her at a train station on her way to join Islamic State militants fighting in Syria and Iraq

Conservative: Severine Ali Mehenni holds a picture of her daughter Sahra dressed in a traditional Islamic robe

Frontline: At least 100 girls and young women from France have left to join ISIS terrorists in Syria (pictured) in recent weeks, up from just a handful 18 months ago when the trip was not even on Europe's security radar

On the day she left for Syria, Sahra strode along the train platform with two bulky schoolbags slung over her shoulder. In a grainy image caught on security camera, the French teen tucks her hair into a headscarf.

Just two months earlier and a two-hour drive away, Nora, also a teen girl, had embarked on a similar journey in similar clothes. Her brother later learned she'd been leaving the house every day in jeans and a pullover, then changing into a full-body veil.

Neither had ever set foot on an airplane. Yet both journeys were planned with the precision of a seasoned traveler and expert in deception, from Sahra's ticket for the March 11 Marseille-Istanbul flight, to Nora's secret Facebook account and overnight crash pad in Paris.

The teenagers travelling to join ISIS come from all walks of life, and although most are first and second generation immigrants from Muslim countries, many come from white French backgrounds.

Despite ISIS repeatedly expressing of deep hatred of Jews, there was even a Jewish girl, according to a security official who spoke anonymously because rules forbid him to discuss open investigations.

Innocence lost: A painting of Mecca is seen on a shelf in Sahra Ali Mehenni's bedroom in France. Sahra is one of 100 girls and young women from France who have left to join terrorists in Syria in recent weeks

Heartbroken: Kamel Ali Mehenni (left) and his wife Severine pose for a picture in their living room in Lezignan Corbieres, France. The couple have spoken to their daughter Sahra three times since she left to join the Islamic State but believe her communication is being scripted by the terrorists

ISIS TRIES TO CUT OFF SAFE EXIT FROM KOBANE: BORDER CROSSING TO TURKEY UNDER ATTACK AS DEATH TOLL 'REACHES 500' A local Kurdish official and Syrian activists say militants from the Islamic State are shelling a Syrian border crossing with Turkey to try and capture it and cut off the embattled town of Kobane. The official, Idriss Nassan, says ISIS aim to seize the crossing in order to close the noose around the town's Kurdish defenders and prevent anyone from entering or leaving Kobane. The Britain-based Syrian Observatory for Human Rights says the militants this morning shelled several areas in Kobane, including the border crossing, which is the town's only gateway to Turkey. The Islamic State pushed into Kobane for the first time since launching its offensive in the area in mid-September. Activists say the fighting over Kobane has killed more than 500 people. Advertisement

Young girls are also coming from elsewhere in Europe, including between 20 and 50 from Britain.

However, the recruitment networks are particularly developed in France, which has long had a troubled relationship with its Muslim community, the largest in Europe.

Distraught families plead that their girls are kidnap victims, but a proposed French law would treat them as terrorists who will be arrested if they return.

Sahra's family has spoken to her three times since she left, but her mother, Severine, thinks her communication is being scripted by the terrorists.

'They are being held against their will,' says Severine, a French woman of European descent. 'They are over there. They're forced to say things.'

The Ali Mehenni family lives in a red-tiled, middle-class home in Lezignan-Corbieres, a small town in the south of France.

Sahra, who turns 18 on Saturday, swooned over her baby brother and shared a room with her younger sister. But family relations turned testy when she demanded to wear the full Islamic veil, dropped out of school for six months and closed herself in her room with a computer.

Distraught: Fouad El-Bahty poses for a photographer in Avignon, France. His sister Nora is one of more than 100 girls from France who have left to join Islamic State terrorists in Syria in recent months

Documentation: A legal file detailing the investigation into the disappearance of French teenager Nora El Bahty, who left her home in Avignon to join the Islamic State in Syria in January

Now she was in a new school. And she seemed to be maturing - she asked her mother to help her get a passport, because she wanted her paperwork as an adult in order.

On the morning of March 11, Sahra casually told her father she was taking extra clothing to school to teach her friends to wear the veil. Kamel stifled his anxiety and drove her to the train station. He planned to meet her there just before dinner, as he did every night.

At lunchtime on March 11, she called her mother. I'm eating with friends, she said.

Surveillance video showed at that moment, Sahra was at the airport in Marseille, preparing to board an Istanbul-bound flight. She made one more phone call that day, from the plane, to a Turkish number, her mother said.

By nightfall, she had not returned. Her worried parents went to police.

They noticed the missing passport the next day.

Terrorist: The teenage girls travelling to join ISIS are forced to pledge allegiance to Abu Bakr al-Baghdadi (pictured), the leader of the Islamist group that has raped and massacred its way through Syria and Iraq

Militant: The teenage girls' departures are less the whims of adolescents and more the conclusions of months of work by organised radical recruitment networks that specifically target young people to join ISIS (pictured)

'Everything was calculated. They did everything so that she could plan to the smallest detail,' Severine says. 'I never heard her talk about Syria, jihad. It was as though the sky fell on us.'

Sahra told her brother in a brief call from Syria that she had married to a 25-year-old Tunisian she had just met, and her Algerian-born father had no say because he wasn't a real Muslim.

Her family has spoken to her twice since then, always guardedly, and communicated a bit on Facebook. But her parents no longer know if she's the one posting the messages.

Sahra told her brother she's doing the same things in Syria that she did at home - housework, taking care of children. She says she doesn't plan to return to France, and wants her mother to accept her religion, her choice, her new husband.

Nora's family knows less about her quiet path out of France, but considerably more about the network that arranged her one-way trip to Syria.

Nora grew up the third of six children in the El-Bahty family, the daughter of Moroccan immigrants in the tourist city of Avignon. Her parents are practicing Muslims, but the family does not consider itself strictly religous.

Not only France: A number of young British Muslims, including Reyaad Khan (left) and Nasser Muthanna (centre) from Cardiff, and Abdul Raqib Amin from Aberdeen (right) have also travelled to Syria to join ISIS

Barbaric: ISIS rebranded as the Islamic State after declaring the foundation of a caliphate earlier this year

She was recruited on Facebook. Her family does not know exactly how, but propaganda videos making the rounds play to the ideals and fantasies of teenage girls, showing veiled women firing machine guns and Syrian children killed in warfare. The French-language videos also refer repeatedly to France's decision to restrict use of veils and headscarves, a sore point among many Muslims.

Nora was 15 when she departed for school on Jan. 23 and never came back.

The next day, Foad, her older brother, learned that she had been veiling herself on her way to school, that she had a second phone number, that she had a second Facebook account targeted by recruiters.

'As soon as I saw this second Facebook account I said, `She's gone to Syria,'' Foad says.

The family found out through the judicial investigation about the blur of travel that took her there. First she rode on a high-speed train to Paris. Then she flew to Istanbul and a Turkish border town on a ticket booked by a French travel agency, no questions asked.

A young mother paid for everything, gave her a place to stay overnight in Paris and promised to travel with her the next day, according to police documents. She never did.

Nora's destination was ultimately a 'foreigners' brigade' for the Nusra Front, an al-Qaida branch in Syria, Foad says. The idea apparently was to marry her off. But she objected and one of the emirs intervened on her behalf. For now at least, she remains single, babysitting children of jihadis.

She has said she wants to come home - and Foad traveled to Syria but was not allowed to leave with her.

'As soon as they manage to snare a girl, they do everything they can to keep her,' Foad says. 'Girls aren't there for combat, just for marriage and children. A reproduction machine.'

Two people have been charged in Nora's case, including the young mother. Other jihadi networks targeting girls have since been broken up, including one where investigators found a 13-year-old girl being prepared to go to Syria, according to a French security official.