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London-born Alexanda Amon Kotey was wanted by the UK and US for his alleged role in the torture and murder of British and American captives.

He was caught as he attempted to cross the border into Turkey as Syrian Democratic Forces pursued the militants in Syria.

"We captured some big commanders. One of them is Alexanda Kotey," Redur Khalil, a spokesperson and senior official in the Syrian Democratic Forces told AFP.

"He was captured by an anti-terrorism unit on 24 January in the countryside near Raqqa. He was trying to escape to Turkey in coordination with his friends and contacts on the Turkish side," he said.

Kotey was part of a four member IS torture cell nicknamed "The Beatles" by their captives due to their accents. They are held responsible for the sickening torture and murder of their hostages.

Mohammed Emwazi - known as "Jihadi John" - was believed to be the executioner of the hostages and was killed in a US drone strike in 2015.  

Aine Davis is being detained in Turkey on terror charges, while El-Shafee el-Sheikh is also believed to be held by the SDF.

Kotey and Sheikh are being questioned by interregators, although Khalil could not confirm Sheikh's capture.

"He is being interrogated. We think that he was a member of a group that has been torturing foreign hostages," the Kurdish official said. 

Syrian Democratic Forces captured Raqqa in October, the one-time centre of power for the militant organisation.

Most of IS' leadership were thought to be based in the eastern Syrian city, where public executions were routine. 

Its fall led to the collapse of IS in Syria, as the last remnants of the once powerful militia fought on in a slither of land along the Euphrates River or fled to Turkey or other parts of Syria.

IS' territories in Iraq were also captured by Baghdad forces late last year.

Agencies contributed to this story.