Nora was a good high school student who dreamed of becoming a doctor. But when she left to wage jihad in Syria, her family soon discovered she had been leading a double life.

The teenager is one of over 1,000 French nationals or residents who have recently travelled to Syria and Iraq to fight alongside extremists -- all from varied backgrounds, dispelling widely-held beliefs that most are disadvantaged, lost youths, experts say.

Nora, from the southern city of Avignon, suddenly departed for Syria in January aged just 15, prompting her elder brother Fouad to launch a desperate search for his little sister.

He discovered she had led parallel lives, owning two Facebook accounts -- one where she talked about her normal teenage life and another where she wrote about her desire to go "to Aleppo to help our Syrian brothers and sisters."

Fouad, who resigned from his job to devote himself to finding Nora, managed to track her down and travelled to Syria in April to meet her for "half-an-hour in the presence of her emir, Omar Omsen," a Franco-Senegalese man.

"I saw her in a bad state, thinner, her face puffy and yellow," he told AFP.

But he was unable to convince her handler to let her go, despite a previous phone conversation when Nora had managed to tell him she wanted to come home. "I am in the midst of hypocrites and cowards who terrorise the Syrians," she had said.

Several days ago, in tears on the phone, she accused her family of abandoning her -- words that have haunted Fouad since.

According to his lawyer Guy Guenoun, the teenager is currently a "hostage". He fears that Nora and other underage girls may be used as "virgins promised to fighters."

- Quick radicalisation process -

Her case is by no means the only one in France, and while the government fears returning jihadists will wage attacks on home soil, some of these are described as people who left purely with humanitarian goals and grew disillusioned with what they witnessed on the ground.

Martin Pradel, the lawyer for several jihadists who have returned to France and are in custody, said the number of departures to Syria exploded in the summer of 2013.

"(Syrian leader) Bashar al-Assad's use of chemical weapons and France's decision not to intervene militarily without the Americans were triggers," he said, referring to accusations the regime had killed hundreds in the Damascus suburbs in August 2013 using chemical arms strikes.

"They had the feeling that their duty was to come to the help of Syrian people."

They spent hours on the Internet with a preference for YouTube and other social networks, looking at shock images and messages marketed by the Islamic State group currently controlling parts of Iraq and Syria, and the Al-Nusra Front, Al-Qaeda's Syrian affiliate.

They did not go to mosques, and drew apart from their family and friends.

Pradel said the radicalisation process was very quick -- "one month" only for one client.

According to Pradel and David Thomson, a journalist and author of a book on French jihadists, these come from all sorts of different backgrounds.

They are young, not so young, from the countryside, from cities, Muslims, people who have converted to Islam -- these represent 21 percent of the total according to the interior ministry -- families, people in stable jobs.

- Waging jihad at home -

And it isn't only those who travel abroad who want to wage jihad.

Myriam, whose real name has been modified to protect her identity, is a law student.

The 20-year-old Muslim is a fervent supporter of the Islamic State group, would like to go live in Saudi Arabia which applies Sharia law, even if she criticises the country for being part of a US-led coalition currently fighting IS.

She told AFP via Twitter that those who cannot go abroad can wage jihad where they are and believes "attacks will take place in France", even if she would not take part in anything of that nature.

She is in contact with friends who went to Syria, all of whom support IS.

"They're very, very happy," she says when asked about those who came back traumatised from what they witnessed.

"It depends on each and everyone's psychological strength in the face of war. I'm a little too human and there are weak people like me who cannot take it. There are people like this in all armed forces in the world. There are quite a few suicides in the US army."

Asked about the beheadings of Westerners carried out by IS militants, she responds that "it is sad, but it's war."

Myriam says she lives in the southwest of France, in "a peaceful and pretty town" in a district with almost no Muslims, and does not go to the mosque.

"There is nothing in my life that could have played a decisive role in my current stand."