Image copyright AP Image caption Kayla Mueller's death was announced in February this year

An American aid worker who was killed while being held hostage by Islamic State (IS) militants, was sexually abused by the group's leader, US officials told ABC news.

Kayla Mueller, 26, was repeatedly raped by Abu Bakr al-Baghdadi. Her family were informed of the abuse in June.

"They told us that he married her, and we all understand what that means," her father Carl told the Associated Press.

Ms Mueller was abducted while working in Aleppo, Syria, in August 2013.

IS said she was killed in a Jordanian air strike in February, but the US blames the group for her death.

Baghdadi personally took the humanitarian aid worker to the home of another senior IS member - Abu Sayyaf - who was in charge of IS oil and gas until his death in a US special forces operation in May, ABC news reports, citing US officials.

US special forces raid

The channel said Baghdadi regularly visited the compound where she was being held and repeatedly assaulted her.

"Kayla did not marry this man. He took her to his room and he abused her and she came back crying," her mother Marsha told AP.

Officials said they had obtained information about the abuse from at least two teenage Yazidi girls who were held hostage as sex slaves and found inside the Sayyaf compound at the time of the US attack.

Image copyright AP Image caption Abu Bakr al-Baghdadi reportedly took Mueller as a "wife"

Ms Mueller was reportedly held for some time by Sayyaf and his wife, Umm Sayyaf, who was also captured by US special forces in May.

At the time, the Pentagon said Umm Sayyaf was suspected of being an IS member and of being complicit in the enslavement of a young Yazidi woman who was rescued in the raid.

Hundreds of young women and girls - many of them Yazidis captured in northern Iraq - are believed to be held as sex slaves by IS militants in areas under their control.

The Yazidi girls provided intelligence used by the US to interrogate Sayyaf's wife, who "spilled everything" about several IS leaders and their whereabouts, a counterterrorism official told ABC.

Who was Kayla Mueller?

Image copyright Mueller family Image caption Kayla Mueller with her mother Marsha before she left for Syria

Kayla Mueller grew up in Prescott, Arizona, with her parents and older brother.

They described her as a woman who devoted her life to helping others. After graduating, she lived and worked with humanitarian aid groups in northern India, Israel and the Palestinian territories.

In 2012 she decided travel to Turkey-Syria border to work for the Danish Refugee Council and Support to Life.

She visited refugee camps and played and painted with Syrian children.

On 4 August 2013, while leaving a hospital run by MSF (Doctors without Borders), she was kidnapped by militants from Islamic State.

In February 2015 they claimed she had been killed by Jordanian air strike, but the US maintain she died at the hands of the group.

Kayla Mueller profile

Mrs Mueller told AP that her daughter had acted as a "mother figure" to the other detainees. "Kayla tried to protect these young girls," she added.

Umm Sayyaf was handed over to the Kurdish authorities in northern Iraq last week to face trial.

The information that has come to light appears to contradict speculation that Ms Mueller was treated well in captivity, as a letter written in 2014 and smuggled out to her family implied.

In it, she said she had been treated with "utmost respect + kindness".