Jihadists are seizing girls from Syrian schools and colleges to serve as "brides", claim activists.

In the town of Raqqa, northern Syria, liberal rebels have been driven out, and fighters from the Islamist groups Islamic State of Iraq and al-Sham (ISIS) hold sway.

Many are foreign-born, and are promised brides by recruiters in return for fighting under the black flag of jihad.

A number of foreign women, sometimes accompanied by their children, have joined the jihadist groups, but men still greatly outnumber them, and some have started to "recruit" brides from the local community, seizing girls against their will.

One of these was 21-year-old Fatima Abdullah, from a tribal area outside Raqqa, whose brother joined ISIS, and promised her to a Tunisian fighter. She resisted, but her family backed the union, and she killed herself with rat poison.

Five activists confirmed the story to the Sunday Telegraph.

One local woman told the newspaper that she had gone to a jihadists' recruiting office to speak to a unit of female jihadists. Among them was a divorced Frenchwoman who had come to Syria with her 12-year-old daughter and four sons to join the militants.

"I went inside their headquarters, which used to be the Christian church," she said. "I asked what the conditions were to join. They said you have to be 18-25, unmarried, and you would earn 25,000 Syrian pounds.

"But if you joined you had the opportunity to marry one of the foreign fighters. However, they make sure you are a real jihadist."

French TV last week broadcast shocking footage of a brigade of about 20 French and 20 Belgian ISIS fighters, who boast of slaughtering "apostates" and film blood-soaked civilian corpses.

Since early January rival rebels groups including western-backed militias loyal to the Free Syrian Army have pushed ISIS back from territory it had taken in northern Syria.

They have succeeded in driving extremists from Idlib province while in Aleppo province ISIS have been forced into towns to the east.

In the town of Azaz, between Aleppo and the Turkish border, ISIS fighters beheaded four prisoners from rebel groups before retreating, and put their heads on a plinth in the middle of the town's main square.

One prisoner, Ahmed Primo, described how he narrowly avoided their fate.

"I heard a voice calling my name for execution," he said. "Then suddenly there was the sound of an explosion. The guards and the emir, the militia leader, were injured, and carried away. The next day the prison was liberated and I escaped."

Primo had been previously held prisoner by the Syrian army loyal to president Bashar al Assad, and was asked whether the treatment he had received from them was worse than that meted out by the jihadists. He said that both forces had beaten him, blindfolded and bound him for weeks at a time, and electrocuted his testicles.

"It is not a question of better or worse. It was exactly the same," he said.

He said that when the uprising began in 2011, he had believed the west would provide military support to secularist rebels, and could not have imagined that country would have been a battleground for brutal factions, many foreign-born.

"When I started out I could never have imagined anything like this," he said. "These people, they do not have our way of life, or of thinking.

It's very strange to us. I didn't expect it would turn out this way."