American volunteer Kayla Mueller who was killed in February while held hostage by Islamic State (ISIS) militants in Syria, was abused by the group’s top leader, officials tell ABC News.

Kayla Mueller, 26, from Prescott, Arizona, was repeatedly raped by Abu Bakr al-Baghdadi, they said.

She travelled to the Turkey-Syria border in 2012 to work with refugees.

Counterterrorism officials made the Food for Life volunteer’s family aware of the abuse in June.

Kayla Mueller was abducted while working in Aleppo, Syria, in 2013. ISIS said she was killed in a Jordanian air strike, but the US blames ISIS for her death.

“We were told Kayla was tortured, that she was the property of Baghdadi. We were told that in June by the government,” Kayla’s parents, Carl and Marsha, told ABC News.

Abu Bakr al-Baghdadi personally took the American aid worker to the home of another senior ISIS member – Abu Sayyaf – who was in charge of ISIS oil and gas until his death in a US special forces operation in May, says ABC News, citing US officials.

The channel said Abu Bakr al-Baghdadi regularly visited the compound where Kayla Mueller was being held and repeatedly assaulted her.

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

Kayla Mueller was reportedly held for some time by Abu 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 ISIS 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 slaves by ISIS militants in areas under their control.

The Yazidi girls provided intelligence used by the US to interrogate Abu Sayyaf’s wife, who “spilled everything” about several ISIS leaders and their whereabouts, a counterterrorism official told ABC.

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 Kayla Mueller was treated well in captivity, as a letter written in 2014 and smuggled out to her family implied.

In that letter, Kayla Mueller tried to reassure her family, saying that she had been treated with “utmost respect + kindness”.