IS widow charged in American hostage Kayla Mueller's death

The wife of a senior Islamic State group (IS) leader who was killed in a US raid last year has been charged in US federal court with holding American Kayla Mueller hostage and with contributing to the aid worker's death, the Justice Department said Monday.

Nisreen Assad Ibrahim Bahar, also known as Umm Sayyaf, admitted after her capture last May that she and her husband kept Mueller, 26, captive along with several other young female hostages, according to an FBI affidavit filed in the case.

US officials have said that while in custody, Mueller was repeatedly raped by Abu Bakr al-Baghdadi, the leader of the Islamic State group.

The criminal complaint, filed by federal prosecutors in Alexandria, Virginia, charges Umm Sayyaf with conspiracy to provide material support to a foreign terror organisation, resulting in death.

The case was brought one year after Mueller was confirmed dead by her family and the Obama administration, though it's not clear when or if Umm Sayyaf will be brought to the US to stand trial.

The 25-year-old Iraqi woman, who was captured last year, is currently in Iraqi custody and facing prosecution there.

Her husband, Abu Sayyaf, a former IS minister for oil and gas, was killed last May in a raid on his compound in Syria by US Special Forces.

"We fully support the Iraqi prosecution of Sayyaf and will continue to work with the authorities there to pursue our shared goal of holding Sayyaf accountable for her crimes," Assistant Attorney General John Carlin, head of the Justice Department's national security division, said in a statement.

Mueller, from Prescott, Arizona, was taken hostage in August 2013, only days after arriving in Aleppo, Syria, to carryout relief work.

Mueller was transferred in September 2014 along with two Kurdish women of Yazidi descent from an IS prison to the Sayyafs, according to the FBI affidavit.

The affidavit adds that the couple handcuffed the captives, kept them in locked rooms, dictated orders about their activities and movements, and showed them violent IS propaganda videos.

After her capture last year, the affidavit says, Umm Sayyaf admitted she was responsible for Mueller's captivity while her husband traveled on IS business.

She said that al-Baghdadi would occasionally stay at her home and that he "owned" Mueller during those visits, which the FBI says was akin to slavery.

The Justice Department complaint echoes earlier assertions from US intelligence officials, who had told Mueller's family that their daughter was repeatedly raped by al-Baghdadi.

"The defendant knew how Ms. Mueller was treated by Baghdadi when Ms. Mueller was held against her will in the defendant's home," the affidavit states.

A Yazidi teenager who was held with Mueller and escaped in October 2014 said also confirmed the abuse Mueller had suffered at the hands of al-Baghdadi.

The circumstances of Kayla Mueller's death remain unclear, as IS claim she was killed in a Jordanian airstrike on the Syrian city of Raqqa, while US officials deny the death of civilians in the airstrike.