TURKEY has captured the sister of slain ISIS leader Abu Bakr al-Baghdadi - calling the arrest an intelligence "gold mine."

Rasmiya Awad, 65, was found hiding in a trailer in the town of Azaz, in north west Syria, Ankara said.

4 Abu Bakr al-Baghdadi's sister, Rasmiya Awad, has been collared by Turkish authorities Credit: Reuters

A top Turkish official said Awad, who is al-Baghdadi's older sister, was living in with her family near the town, which is part of the region administered by Turkey after its recent invasion of northern Syria.

The official said the sister was with her husband, daughter-in-law and five children.

The adults are being interrogated, he said, speaking on condition of anonymity.

4 Abu Bakr al-Baghdadi was killed in Iraq Credit: AP:Associated Press

4 A man, believed to be Rasmiya Awad's husband, was also captured Credit: Reuters

4 As was a woman believed to be their daughter-in-law Credit: Reuters

The official added: "This kind of thing is an intelligence gold mine.

"What she knows about (IS) can significantly expand our understanding of the group and help us catch more bad guys."

Al-Baghdadi, an Iraqi from Samarra, was killed in a US raid in the nearby province of Idlib last month.

The raid was a major blow to the group, which has lost territories it held in Syria and Iraq in a series of military defeats by the US-led coalition and Syrian and Iraqi allies.

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Many IS members have escaped through smuggling routes to northwestern Syria in the final days of battle ahead of the group's territorial defeat earlier this year, while others have melted into the desert in Syria or Iraq.

The reclusive leader al-Baghdadi was known to be close to one of his brothers, known by his nom de guerre Abu Hamza.

Al-Baghdadi's aide, a Saudi, was killed hours after the raid, also in northwestern Syria, in a US strike.

The group named a successor to al-Baghdadi days later, but little is known about him or how the group's structure has been affected by the successive blows.