Turkey has captured the elder sister of Abu Bakr al-Baghdadi, the slain leader of the "Islamic State" (IS) group, according to a senior Turkish official.

The official said the 65-year-old known as Rasmiya Awad was captured in a raid on Monday on a trailer container she had been living in with her family in Azaz, in northeastern Syria. The official called the arrest "an intelligence gold mine."

Read more: IS declares new leader after death of al-Baghdadi

"What she knows about [IS] can significantly expand our understanding of the group and help us catch more bad guys," the official said.

Azaz is a town in a region that is administered by Turkey. Allied Syrian forces manage the area, known as the Euphrates Shield zone.

Awad was with her husband, daughter-in-law and five children at the time of the arrest. The adults are being interrogated, the official said.

Turkey boasts of 'strong counterterrorism cooperation'

Fahrettin Altun, the communications director for Turkish President Recep Tayyip Erdogan, said the woman's capture was evidence of Turkey's determination to fight against IS.

"The arrest of al-Baghdadi's sister is yet another example of the success of our counterterrorism operations," he wrote on Twitter early Tuesday.

"Much dark propaganda against Turkey has been circulating to raise doubts about our resolve against Daesh," he wrote, using another name for IS. "Our strong counterterrorism cooperation with like-minded partners can never be questioned."

Little is known about the reclusive leader's sister. Al-Baghdadi was known to be close to one of his brothers, known as Abu Hamza.

Al-Baghdadi was killed in the nearby province of Idlib during a US raid last month. The raid was a major blow to the militant group, which has lost most of its territory in Syria and Iraq after a series of military defeats by a US-led coalition and Syrian and Iraqi allies.

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dv/cmk (AP, Reuters)

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