Despite gradually losing its territories in Syria and Iraq, the Islamic State (Isis) is tightening its grip on women it is holding as sex slaves. Around 3,000 Yazidi girls, some as young as 12, are being sold like chattel on smartphone apps. The advertisements carry the photographs of women and a price tag for each.

The militants post details primarily on Telegram and sometimes on Facebook-owned WhatsApp too, along with the names of the women's "owners", an activist told the Associated Press on the condition of anonymity fearing for his safety.

One advertisement on Telegram describes a girl who is up for a sale as: "Virgin. Beautiful. 12 years old... Her price has reached $12,500 (£9,690, €11,307) and she will be sold soon."

The families of women who are kept as slaves pay smugglers to get them out of Isis captivity. However, when anyone tries to rescue them, they are reportedly killed by the jihadist group. Mirza Danai from an aid organisation said it has not been easy for captives to escape and the situation has become more dangerous in the last three or four months.

The Isis keeps a tab on every slave in its captivity and if a girl escapes from her owner, they get to know as each girl is registered at every checkpoint of the group.

When Lamiya Aji Bashar tried to escape from the clutches of the group, the fighters raced after her and activated a land mine, which killed her companions – an eight-year-old girl and a 20-year-old. The explosion also left her blind in her right eye and caused severe burns to her face.

A posting for a mother of two shared on WhatsApp, which the news agency saw, reads: "She wants her owner to sell her." She was given a price tag of $3,700.

"We have zero tolerance for this type of behaviour and disable accounts when provided with evidence of activity that violates our terms," said Matt Steinfeld, a spokesman for WhatsApp. He also asked users to report if they come across such messages circulating on the app.

Since both WhatsApp and Telegram use end-to-end encryption to protect users' privacy, they themselves cannot access users' content.

Speaking for Telegram, its spokesperson Markus Ra told AP that his company works to prevent abuse of its service and routinely removes public channels used by the Isis.

"Telegram is extremely popular in the Middle East, among other regions. This, unfortunately, includes the more marginal elements and the broadest law-abiding masses alike," he said.

The population of the Yazidis was estimated at 500,000 before war broke out in Iraq, but their number remains unknown today.

"Daesh [Isis] is proud of what it's done to the Yazidi's. They are being used as human shield. They are not allowed to escape or flee. Probably they will be assassinated. Where is the world in all this? Where is humanity?" an escapee named Nadia Mourad, told the US Congress and the European Parliament when she appealed to them for international help.