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

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

Counter-terrorism officials made her family aware of the abuse in June.

Mueller was abducted while working in Aleppo, Syria, in 2013. IS said she was killed in a Jordanian air strike, but the US blames IS 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," her parents, Carl and Marsha, told ABC News.

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, citing US officials, reports.

The channel said he regularly visited the compound where she 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 sex slaves and found inside the Sayyaf compound at the time of the US attack.

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.

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

In it, Mueller tried to reassure her family, saying that she had been treated with "utmost respect + kindness".

The humanitarian aid worker from Prescott, Arizona, travelled to the Turkey-Syria border in 2012 to work with refugees.