The report noted that the detained Europeans include people who joined the Islamic State from countries such as France, the Netherlands and Poland, with an unspecified number already released in coordination with European embassies.

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“They're from everywhere, Europeans and Arabs,” Ghabi said. “We tried to rehabilitate them and alter their state of minds. Those who wished to return home were allowed to call their embassies and coordinate with them through us.”

The report, which could not be independently verified, highlights some of the unexpected and potentially thorny issues that are arising as the Islamic State’s self-declared caliphate in parts of Iraq and Syria continues to fall apart. The extremist group, also known as ISIS and ISIL, has lost huge amounts of territory to an array of factions in both countries that are backed by nations including the United States and Turkey.

Syrian rebels and analysts say that Islamic State members are increasingly being captured in battle or abandoning the “caliphate.” That, in turn, raises the possibility that men, women and children who had been living in the group’s strongholds could get further caught up in the lawless brutality of the Syrian conflict.

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Just about every group in Syria — religious extremists, moderate rebels and Syrian President Bashar al-Assad’s government — rejects the Islamic State for its brutal methods.

Assad’s forces have been accused of systematic human rights violations since the conflict began in 2011. Rights organizations also say that Syrian rebel forces have imposed harsh forms of Islamic law, and have committed acts of torture and other human right violations.

Last summer, video footage purported to show a child being beheaded by fighters from a rebel group that had briefly received weapons under a U.S.-supported program to arm Assad's opposition.

Although it did not gain access to the detention facility run by Jaish al-Tahrir, the BBC said it obtained material from inside. Prisoners cited decent conditions. One man who left his home in the Netherlands to join the Islamic State is quoted as criticizing the extremist group’s treatment of foreign (presumably, non-Arab) nationals. “They treat us very bad, especially people from another country,” the unidentified man said.

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The report cited a rise in the number of people fleeing Islamic State territory because of Turkey’s offensive in northern Syria, called Operation Euphrates Shield. That operation started over the summer and deprived Islamic State militants of their last access to the border with Turkey, a critical lifeline for people and weapons.

The Islamic State, Ghabi told the BBC, “has been falling apart for the past seven or eight months, according to the defectors we spoke to. However, Operation Euphrates Shield further degraded IS and led to its dismemberment following the rapid advances of our forces.”

The report also cited the creation of an “underground railroad” involving rebel groups and Western intelligence services that helps people flee Islamic State areas.

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Fighters living in the group's de facto capital of Raqqa, a city in eastern Syria, have been sending messages to rebels asking for assistance in fleeing, the report said. Syrian activists also say they have noticed a sharp rise in recent months of people in Islamic State strongholds, including European nationals, inquiring about ways to escape. Those caught trying to leave the group’s territory face severe punishment, including execution.

Western intelligence agencies fear that Europeans who have joined the militants are trying to return to their countries of origin to carry out terrorist attacks.