Child marriage has boomed in the Libyan city of Derna since it was taken over by the Islamic State (Isis) group, with girls as young as 12 married off to jihadist fighters, often in exchange of protection and power, local doctors have confirmed.

The number of under-age girls forced to marry has increased 15 fold since an IS local branch seized large part of the coastal city in October last year, local doctors told The Times.

"Just in the clinics that we are able to monitor, we are seeing four to five cases of under-age brides every week and it's getting worse," Asmaa Said, a local activist who drew up a report on the practice based on data secretly passed on to her by doctors operating in Derna, told the British newspaper.

"There is also the spread of STDs and the growing prevalence of miscarriages, premature and stillbirths".

Said brought the account of a 12-year-old schoolgirl who was married off to a jihadist and suffered irreparable damage to her reproductive organs after a miscarriage that followed repeated rapes by her husband.

The distressing situation many children are facing in Derna was confirmed by local gynaecologists who said they are treating sex-related injuries in an increasing number of girls with no knowledge of sex.

"They come into the clinic playing with their dolls," the doctor, who preferred to remain anonymous, told The Times.

Another Derna activist, named only as Abdel-Rahman, said parents were in many cases giving away their child daughters to fighters, mainly foreigners from Arabic-speaking countries, as marital status is perceived as a form of protection for them and by reflex for the whole family.

"Some of them get nice cars and nice houses, too," he told the newspaper.

Libya has been embroiled in fighting since the overthrow of late dictator Muammar Gaddafi in 2011.

Battling for control of the country currently involves pro-government forces, Libya Dawn, an umbrella group including radical and moderate Islamists, and an IS local offshoot that recently infiltrated several coastal cities.

In the areas it controls, IS enforces a reign of terror based on its strict interpretation of Sharia law.

Over the last few months, the extremist group has released two separate videos showing the beheading of 21 Coptic Christians and of 30 Eritrean migrants on a beach near Tripoli.

Under the jihadi group's rules, purported un-Islamic activities, such as smoking and playing instrumental music, are also banned.