Yazidi women and children are being bought back from their Islamic State (Isis) captors in secretive deals involving Muslim middle-men operating across borders in Syria and Iraq, IBTimes UKcan reveal.

IS (Daesh) sells the young boys and girls for between $2,000 (£1,340) and $20,000 (£13,400) and until recently the Kurdistan Regional Government (KRG) was paying the ransoms. The KRG has since stopped paying, citing financial constraints, but said it is committed to working to free all Yazidi captives held by IS.

However, Yazidi families are still paying for the release of their loved ones, either by raising bank loans in Iraqi Kurdistan or by relying on wealthy individuals. In most cases, they are provided with pictures of the captives – examples of which have been seen by IBTimes UK – and given a price depending on the age and sex of the victims.

"When Yazidis buy our beloveds back from Isis, we don't deal directly with them – it will be undercover through a Muslim smuggler," Maher Nawaf, who works with the NGO Yazda, said. "The costs at the beginning of the crisis were about $2,000 or $3,000, but it is a lot more than that now."

The girls will be valued according to how young they are and their beauty
- Maher Nawaf, NGO Yazda

Ameena Saeed Hasan, a former Iraqi MP who now works to help free those still inside IS territory in Iraq and Syria, said freeing Yazidi captives – thousands of whom were enslaved by IS after it surged across northern Iraq in 2014 – was getting harder since the KRG stopped paying.

"In the beginning, there was just a few women and children rescued – so it was not so much money. Now, more than 2,000 people were rescued, and the KRG have helped the families pay for most of them," she said.

Hasan has been personally involved in freeing several hundred women and children, and estimates that as many as 2,500 have been rescued, while 3,000 remain captive inside IS. As many as half of this number are thought to be children, and it has become harder to get them out in recent months, she claims.

"The KRG were helping the families free the hostages by paying for them. But now they are saying to the families – you pay for them yourself and we will give you money after to pay you back. They say they don't have the money any more," Hasan said.

Families of those still inside IS say this change of tactic has been disastrous – as the terrorist group believes they are still helping, and costs have consequently inflated – though many within the networks from the Yazidi side work for free. The KRG confirmed it had previously paid ransoms but that the programme had been halted because of "financial constraints".

Negotiations with the fighters holding three Yazidi children in Raqqa broke down several months ago and they remain inside Syria, according to the smugglers hoping to rescue them. In pictures seen by IBTimes UK, the three children are seen in a bare concrete room. The older boy is 12 years old, the younger girl is 10 and a toddler holding a rifle is three.

Their ages make them strategically valuable to the group and consequently expensive. "After the ages of 10, the boys will be recruited to the Isis camps – and once they are there, it is more difficult to reach them," Nawaf explained. "The girls will be valued according to how young they are and their beauty."

Girls tend to lose their value the longer they remain captive, while boys increase their skills and value to the group as they graduate through cub camp training.

Where the captives are also makes a big difference to the cost. If they are deep inside Syria, it is more likely they will have to be bought directly from a fighter or his family, and their smugglers will need to be paid as well, while for those held in Iraq it has been possible for women and children to escape directly to the smuggler.

"If they are near to the front lines, then they are cheaper as they can be reached easily, but if they are very far inside IS, it is very dangerous and costs a huge amount of money," Nawaf said.

Murad Haji, a doctor from Sinjar living in Duhok, managed to buy back his wife and young son back from the IS capital of Raqqa after paying $20,000 to the fighter who bought them for the equivalent of $50.

Negotiations for their freedom went on for several months and were only made possible because he could borrow from banks in Duhok and Erbil, and family members abroad – an option not open to the majority of those displaced in an array of camps outside Duhok. "I was a dead man alive without my wife and child," he told IBTimes UK. "They are worth my whole world and all my life. The world has forgotten our people. Now no one is helping us at all – so what else can we do to rescue our families?"

How IS makes its money

Isis fighters stand on confiscated cigarettes before setting them on fire in the city of Raqqa Reuters

IS has a variety of income streams in the territory that it controls in Iraq and Syria, not least its oil wells, which are believed to bring in £33m per month, and the smuggling of antiquities looted from museums and historic sights.

It also makes millions of dollars in taxes on Syrians and Iraqis it controls, although that has lessened since the Iraqi government halted paying salaries to government workers living under IS rule in Mosul.

Both the US and Russia have called for more to be done to prevent IS from making money in the areas it controls, with the later blaming Turkey for turning a blind eye towards smuggling across its porous border.

But stemming the flow of IS funding by bombing risks alienating Iraqis and Syrians living in Sunni areas under terrorist control by destroying their livelihoods, a difficult prospect as the West tries to persuade civilians and power-brokers in Iraq and Syria that IS is not the only option for Sunnis.