WASHINGTON - Turkey captured the sister of dead Islamic State leader Abu Bakr al-Baghdadi on Monday in the northern Syrian town of Azaz, a senior Turkish official told Reuters, and is interrogating her husband and daughter-in-law who were also detained.

Rasmiya Awad, 65, was detained in a raid near Azaz, the official said, referring to a Turkish-controlled Syrian town near the border. When captured, she was also accompanied by five children.

“We hope to gather a trove of intelligence from Baghdadi’s sister on the inner workings of ISIS,” the official said.

Little independent information is available on Baghdadi’s sister and Reuters was not immediately able to verify if the captured individual was her. Baghdadi killed himself last month when cornered in a tunnel during a raid by US special forces in northwestern Syria. Islamic State, in an audio tape posted online on Thursday, confirmed that its leader had died and vowed revenge against the United States.

Turkish President Tayyip Erdogan’s communications director said the woman’s capture was evidence of Turkey’s determination to fight against Islamic State. “The arrest of al-Baghdadi’s sister is yet another example of the success of our counter-terrorism operations,” Fahrettin Altun wrote on Twitter early on Tuesday.

“Much dark propaganda against Turkey has been circulating to raise doubts about our resolve against Daesh,” he wrote, using another name for Islamic State. “Our strong counter-terrorism cooperation with like-minded partners can never be questioned.”

Baghdadi had risen from obscurity to lead the ultra-hardline group and declare himself “caliph” of all Muslims, holding sway over huge areas of Iraq and Syria from 2014-2017 before Islamic State’s control was wrested away by US-led coalition forces including Iraqis and Syrian Kurds.

The group, also known as ISIS, said a successor to Baghdadi identified as Abu Ibrahim al-Hashemi al-Quraishi had been appointed. A senior US official last week said Washington was looking at the new leader to determine where he came from.

World leaders welcomed his death, but they and security experts warned that the group, which carried out atrocities against religious minorities and horrified most Muslims, remained a security threat in Syria and beyond.

Meanwhile, Turkish President Tayyip Erdogan said the Kurdish YPG militia had not withdrawn from some Syrian border areas and that US forces were still carrying out joint patrols with the group, contrary to an agreement between them.

He was speaking as Turkish and Russian soldiers in armoured vehicles held their second joint ground patrol in northern Syria near the town of Kobani, under a deal to push the YPG some 30 km (19 miles) away from Turkey’s border. Nearly a month ago, Turkey and Syrian rebel allies launched a cross-border incursion against Kurdish YPG fighters, seizing control of 120 km (75 miles) of land along the frontier.

Turkey subsequently struck two separate deals with the United States and Russia for the YPG to withdraw from the “safe zone” it plans to form in the region, in return for Ankara stopping its offensive against the group. While Washington and Moscow have said the fighters had left the border region, Erdogan said this was not the case.

“These areas are not cleared of terrorists. Terrorists have not been taken out of either Tel Rifaat or Manbij,” Erdogan said, referring to two towns in the western border area where he said YPG fighters remained.

They were also still present east of Ras al Ain, a town Turkey targeted in the incursion, Erdogan said in parliament. He said Turkey would abide by the deals as long as Washington and Moscow kept their promises. He later told reporters that US forces were still holding joint patrols with the YPG inside the 30-km border strip from which the militia was meant to withdraw.