Kayla Jean Mueller

US President Barack Obama yesterday confirmed the death of Kayla Jean Mueller -- who was taken hostage by the Islamic State group in Syria -- and vowed to hunt her captors.

"No matter how long it takes, the United States will find and bring to justice the terrorists who are responsible for Kayla's captivity and death," he said in a statement.

The 26-year-old Arizona aid worker was seized in August, 2013 in Aleppo.

Last week ISIS claimed she had been killed in a Jordanian air strike in the Syrian city of Raqa, the militant group's proclaimed "capital."

That account was treated with skeptisim, but over the weekend ISIS sent Mueller's parents a "private message" with "additional information", that allowed the intelligence services to confirm her death, the White House and family said. The Washington Post reported that Mueller's family had been sent a photograph of their daughter's body.

Mueller was believed to be the last US hostage being held by ISIS, following the execution of AFP contributor James Foley and American-Israeli journalist Steven Sotloff.

Mueller's Parents Carl and Marsha Mueller voiced their heartbreak at the death of their daughter, but said they were proud of her and the humanitarian work she did.

In a letter from captivity in early 2014 -- which was partially redacted -- Muller said she was "completely unharmed" by her captors but heavy-hearted about the toll it must be taken on her family.

"Just the thought of you all sends me into a fit of tears," she wrote in the letter released by her family.

She dreamt about family camping trips and imagined meeting them at the airport if she were released.

Obama praised Mueller's humanitarian work at home, in the Middle East and elsewhere.