Sophie Kasiki stared at the photograph of a young English-speaking boy in a camouflage uniform and black bandana covered in Arabic calling for unbelievers to be killed in the latest Islamic State propaganda.

Her eyes welled and she swallowed hard. “That could have been my son,” she said, her firm voice wavering. “That’s hard for me to say and makes me want to cry. I would have killed us both rather than let him become a killer, rather than let him fall into the claws of those monsters.”

The “monsters” she is referring to are Islamic State, and Kasiki weighs her words; she knows her four-year-old son was only ever at risk of falling into the jihadis’ lair because she had taken him there.

Kasiki is one of the few western women who have been to the capital of the Isis-declared caliphate at Raqqa in Syria and returned to recount the tale. It was, she said in her first interview with a British newspaper, like a journey into a hell from which there seemed no return.

“I have felt so guilty. I have asked myself how I can live with what I have done, taking my son to Syria,” she told the Observer. “I have hated those who manipulated me, exploited my naivety, my weakness, my insecurity. I have hated myself.”

About 220 French women are thought to be with Isis in Iraq and Syria, according to the country’s intelligence services. Two years ago only 10% of those leaving France to join the jihadis were women. Today the proportion is 35%. A third are converts, like Kasiki. Her story, Dans la Nuit de Daech (In the Night of Daesh), published by Robert Laffont Editions, reads like a thriller.

In the night of Daech, recounts Sophie Kasiki’s journey to Raqqa, life in captivity and then escape.

Kasiki, 34, a petite but fiercely determined woman with neatly braided hair (who will not give her real name for fear of Isis reprisals), seems an unlikely recruit to the Islamist cause. Born in the Democratic Republic of Congo, and raised in a fervently Catholic and comfortable household of strong, independent women, she was nine when sent to live with her older sister near Paris after their mother died. The death of the woman she still calls her “guardian angel” sparked a childhood depression that cast a long shadow into adolescence and adulthood; a “hole in the heart” that even a happy marriage and motherhood failed to close.

While employed as a social worker helping mainly immigrant families in the Paris suburbs, Kasiki decided to convert to Islam, without telling her fervently atheist husband, believing it would fill the gap in her life. Her new faith brought only temporary psychological comfort, but introduced her to three Muslim men, 10 years her junior, whom she nicknamed Les Petits (the little ones) and teased like younger brothers.

In September 2014, the three disappeared, later turning up in Syria, from where they maintained daily contact with Kasiki. She saw herself as a conduit between three lost boys, who simply needed to know their mothers were missing them to catch the next plane home, and their distraught families. Slowly the roles reversed. “I thought I was in control of the situation, but I realise now they were probably trained to recruit people like me,” she said. “Little by little they played on my weaknesses. They knew I was an orphan and I had converted to Islam, they knew I was insecure …”

On 20 February 2015, Kasiki told her husband she was travelling to work in an orphanage in Istanbul for a few weeks and taking their son. Instead she took the well-worn jihadi route to southern Turkey and into Syria.

Installed in the Isis stronghold of Raqqa, the reality of daily life was predictably different from the “paradise” painted by her hometown friends. Kasiki was ordered not to go out alone and only then covered from head to toe, to hand over her passport, and to limit communications with her family in France.

At the city’s Isis-run maternity hospital, where she was to work, she was shocked by the squalid conditions, staff indifference to patients’ suffering, and a hierarchy in the city that put “arrogant foreign fighters” at the top of the social heap and Syrians at the bottom. The family apartment Kasiki was allocated had been hastily abandoned by its Syrian owners and their caged canaries served as an increasingly potent metaphor for her and her son’s confinement.

It took just 10 days for Kasiki to wake from what she describes as a “paralysing torpor”, prompted by regular missives and family photos emailed by her desperate husband, and to realise her terrible mistake.

“I asked to go home. Every day, I said I missed my family and my son needed to see his father. To begin with they made excuses, then came the threats. They said I was a woman alone with a child and I couldn’t go anywhere, and if I tried to leave I would be stoned or killed.

“I was terrified someone would come and take me to prison and I’d have to leave my son with them. I spoke to him all the time: I tried to impress on him things he wouldn’t forget; that his father and I loved him; that he had to be kind to girls. I talked in the hope it would sink in, and if something happened to me and he fell into Daesh’s clutches he would have my voice in his head and would not be able to kill … I was like a lioness trying to protect him.”

When one of the Frenchmen demanded to take the boy to pray at the mosque, she snapped: “Keep your hands off my boy.” The response was a punch in the face. “I was in a foreign city where I knew nobody and didn’t speak the language. I looked at my son and knew that I had made a monumental mistake, the worst of my life. I knew then I had to be strong and do everything possible to get him out of there.” The Frenchmen took Kasiki and her son to the madaffa (guest house), a prison in all but name and home to dozens of foreign women, where she was shocked to see young children watching Isis decapitations and killings on television while their mothers cheered and clapped. “The women saw Isis fighters as their Prince Charming, someone who was strong, powerful and would protect them. The only way out of the madaffa was to marry one. In reality, these western women were just wombs to make babies for Daesh.”

The following day, while her jailers were organising a marriage, Kasiki discovered an unlocked door and walked out. She kept walking.

Her account of her escape from Raqqa is the edge-of-your-seat stuff of thriller movies. After being taken in by a local family, who risked their lives sheltering them, Kasiki made contact with Syrian opposition fighters, mobilised by her husband in France. On the night of 24 April 2015, a young Syrian took Kasiki, with her son hiding under her niqab, by motorbike to the Turkish border. Had they been stopped at a checkpoint or caught fleeing, all would have faced death.

In Paris, Kasiki was interrogated by French intelligence officers, remanded in jail for two months and prevented from any contact with her family. Today she and her husband are reconciled, but she is still facing possible charges for child kidnapping.

“I have gone back over everything and asked myself, how did this happen, how could I have done this? Yes, I was naive, confused, fragile, vulnerable even, but how were these ordinary, not particularly smart boys intelligent enough to brainwash me? It is a question I still ask myself.”

Kasiki knows that she has had an improbably lucky escape, one that many western girls and women who have been lured by the siren calls of Islamic State and trapped in Syria will never enjoy.

After her return to France, her husband showed her a photo Isis had sent him of their son posing with an automatic rifle. “It must have been taken while we were there, but it was the first time I’d seen it. I felt sick to the heart,” she said.

“I will always feel bad about taking my son into this hellish nightmare, so bad I often feel completely paralysed with guilt. But I have to be strong and go on. The most difficult part is over. We have escaped from the clutches of these people and we are alive.

“Now I must prevent other people being drawn into this horror. What can I say? Don’t go.”