Former fighters from the Islamic State (Isis) and their families are being held in a secret internment camp controlled by Syrian rebel fighters. Rebel group Jaysh al-Tahrir say that they have apprehended 300 defectors and combatants, including many European citizens, before imprisoning them at the covert camp in a village in northern Syria.

The operation to fight Isis has led to a surge in fleeing fighters, with Jaysh al-Tahrir commander Mohammad al-Ghabi mopping up the defectors and jailing them. Al-Ghabi says that some foreign fighters have been repatriated after they made contact with their embassies.

He said that at least half a dozen foreign fighters have left and are now facing imprisonment back in Europe. The foreign fighters include Dutch and Polish nationals, as well as those from North Africa and across the Middle East and Central Asia.

Those who did not want to return to their home countries, or were suspected of committing crimes in Syria, would stand trial in a Sharia court and be subject to Islamic law. Al-Ghabi added that of some of those found guilty could be executed.

Men, women and children are held in the rural camp with the aim of rehabilitating those who had lived and and fought under the jihadists, al-Ghabi claimed. "We tried to rehabilitate them and alter their state of minds," he told the BBC. "Those who wished to return home were allowed to call their embassies and co-ordinate with them through us."

Rebel group Jaysh al-Tahrir operates mainly in the Hama and Aleppo Governances and was formed by five units of the Free Syrian Army. Vetted and supplied by the CIA they were intended as a moderate buffer to more radical groups such as the al-Nusra Front – now Jabhat Fateh al-Sham, according to the report.

They have been fighting alongside Turkish-backed Islamic groups as Syria's northern neighbour turns up the heat on Isis under Operation Euphrates Shield, the Long War Journal reported. It is this operation that has led to the collapse of Isis, according to al-Ghabi.

"Isis has been falling apart for the past seven or eight months, according to the defectors we spoke to. However, Operation Euphrates Shield further degraded Isis and led to its dismemberment following the rapid advances of our forces," he added.

One former Isis fighter named Abu Sumail, who travelled from the Netherlands to Turkey before entering Syria. He said he did not like living in the self-declared caliphate: "It's very hard for us to live there — it's not our lifestyle because we are used to a lot of things and then we come there and they directly start to treat you hard.

"You give your life to them, so they are going to start to take control of your life. They use you for bad stuff".

The BBC added an underground railroad is being developed in the country together with other rebel groups and British and European intelligence services in an attempt to find, capture and return Isis supporters.