(CNN) Sitting in front of his high-end computer, staring intently at the screen, chunky headphones on and mouse busily clicking away, Omar Omsen looks like a regular office worker.

Only the camouflage t-shirt and the tent walls billowing around him offer a hint that this is no ordinary workplace, but the epicenter of jihadi recruitment in Syria.

The clip he's working on is not a pop video or newsreel but a piece of radical Islamist propaganda, in praise of the "Charlie Hebdo" attackers who killed 12 people in Paris in January 2015.

Omsen -- a.k.a. Omar Diaby -- is France's "super jihadist."

Omar Omsen leads a French "katiba," or battalion, in Syria. He was radicalized while in prison.

Through his series of online videos, released under the name "19HH" (a tribute to the 19 perpetrators of the September 11 attacks), French authorities say he is responsible for recruiting about 80% of French-speaking jihadis heading to Syria and Iraq.

The clips fuse Hollywood special effects, rap music, religion and conspiracy theories in an attempt to convince young French Muslims to join the fight.

Among his followers, who listen rapt as he preaches with messianic fervor, Omsen is treated as a spiritual leader.

JUST WATCHED Jihadist tech tools revealed Replay More Videos ... MUST WATCH Jihadist tech tools revealed 02:36

"All the guys were looking at him like he was god, like it was a sect," says Fouad El Bathy, who has seen Omsen in action up close in Syria.

"He made me think of a guru -- they were venerating him."

El Bathy was in Syria to try and rescue his baby sister, Nora, who ran away to Syria when she was just 15 years old, one of the thousands who have made the journey.

Hotbed of recruitment

And US National Intelligence Director James Clapper says the number is now even higher. In February, he told the Senate Armed Services Committee "more than 36,500 foreign fighters -- including at least 6,600 from Western countries -- have traveled to Syria ... since the conflict began in 2012."

And while the promenades of southern France may seem a world away from the war zone, the region -- and Omsen's childhood home, Nice, in particular -- has become a hotbed of jihadi recruitment.

Local imam Boubekeur Bekri says he knows people who "have been attracted by this massive lie of paradise."

JUST WATCHED Report: Number of ISIS foreign fighters increases Replay More Videos ... MUST WATCH Report: Number of ISIS foreign fighters increases 03:55

"They are transformed in a few weeks, or even in a few days -- it's like a bomb goes off ... ISIS can be very persuasive," he explains. "They take fragile people, and make them more fragile, and then they promise them paradise. They work to alienate and isolate these people."

Radicalization, he says, is "like a virus ... When a virus infects a lot of people, it's a pandemic and you can't use regular pills to cure it. You need bigger resources."

But despite a supposed crackdown -- France has been in a state of emergency since the Paris attacks in November last year -- the country's jihadi exodus is continuing unabated.

According to the latest Interior Ministry figures obtained by CNN, the number of French nationals caught up in jihad have jumped 13% over the past six months: between May and July this year alone, 67 more people became involved.

Europol says "a significant percentage of all foreign terrorist travellers in Syria/Iraq are now female ." Most women making the trip are married to fighters soon after they arrive, and "historical data suggests women are less likely to (be able to) return than men."

State of shock

When Nora disappeared from her home in Avignon two years ago, her family had no idea where she had gone.

"It was 6.30pm, 7.30pm and she was not at home," says El Bathy. "We were worried, since this was not her way, she always came home and did her homework." He began a frantic search for his much-loved youngest sibling.

Teenager Nora El Bathy ran away from home after being radicalized by Omar Omsen.

"I went around the neighborhood, and there was no sign of her. I went to the train station and she was not there, to the town center and it was the same ... I called the hospital, the clinic, there was no news. I went to the police station, nothing."

Eventually, El Bathy recalls "She spoke to me on Facebook. She said 'How are you?' and I said 'What do you mean how are you? I'm not okay! Where are you?'

"'I'm in Syria, thank God,'" his sister told him. "'I'm finally going to help Syrians.'"

"We were in a state of shock," El Bathy says. "My sister spoke like she was in heaven, like she had reached her goal. Finally, she thought, she was going to have a role on earth. My mother fainted."

Radicalization attempts

It wasn't until much later that he realized Nora had been taken in by the notorious "super jihadist."

"I heard about Omar Omsen in the media -- I didn't know who it was," El Bathy says. "He gave an interview to a journalist and he was speaking about Nora. He said that my sister was like his own daughter."

JUST WATCHED Jihadi 101: The books at the core of ISIS recruitment Replay More Videos ... MUST WATCH Jihadi 101: The books at the core of ISIS recruitment 03:49

El Bathy decided he had to follow in Nora's footsteps, to risk it all and travel to Syria and try to bring his sister home to France.

When he arrived, he was put into a pick-up truck to be driven "like crazy" to the group's compound: "I thought I was going to die on my way there."

"I saw a Kalashnikov right next to me -- that's when I realized it was serious," he says. "You never see weapons so close to you.

"There was a Syrian guy called Abu Khalid, he was the one driving. He said to me. 'You should fight, you should save Syrians, and fight for Allah' -- he was trying to radicalize me."

And the radicalization attempts didn't stop when he reached the group's shared home, where he was forced to sit and listen while Omsen preached jihad. "I could feel he was trying to manipulate me," El Bathy says. "What he was saying was too good to be true."

Under surveillance

Eventually, El Bathy was allowed to visit Nora -- accompanied by Omsen.

"We walked toward my sister's house ... We stopped at a shop and bought candies for my sister, because she loves candies. Omar said 'I love your sister like she was my own. It's because of me that she's not married to a fighter.'

JUST WATCHED ISIS member recruits undercover journalist as jihadi wife Replay More Videos ... MUST WATCH ISIS member recruits undercover journalist as jihadi wife 03:32

"He told me to get in [to the room where Nora was being kept], and my sister jumped on me. We couldn't stop hugging each other. She kissed me here, here, here -- she even kissed me on the mouth by mistake," he remembers, sobbing and smiling.

"I told her 'Let's go home.' She said 'I can't, I can't.' She knocked her head against the wall of the room. I saw there was a CCTV camera monitoring us, and I understood ... She wanted to come back but Omar stopped her from coming with me."

El Bathy, who works for a deradicalisation program and filmed his trip to Syria on hidden cameras, says he spent several days living among Omsen's followers, and was surprised to find that Nora was not the youngest there.

"There were more than 50 minors where I was. There were 5, 6, 7, 8, 10 year-old kids, all ages," he says. "They were from all around the world."

Armed escort

But he was not permitted to spend much time with his sister: "I could only see my sister twice. The last time I could only give her a bit of money to keep in touch with us -- I gave maybe 200 euros and a phone, but they took everything away."

El Bathy says Omsen and those around him suspected that he planned to "kidnap" Nora and take her out of the country; as a result, he says "I was held in the villa with an armed man."

In the end, he says, "they told me to go back to France," leaving him little choice but to go home alone.

French President Francois Hollande says the country is at war , but with terror attacks at home and jihadi converts abroad, it is increasingly a battle within itself -- and families on both sides -- those of the victims and the perpetrators -- are suffering.

And even as El Bathy and others like him bravely speak out, Omsen and his propaganda machine continue to lure men and women to wage war at home and abroad.