Washington, US: US aid worker Kayla Mueller, held hostage by IS militants for 18 months, is dead, her family said on Tuesday, but the circumstances were unclear and US President Barack Obama vowed to hunt down the culprits.

Mueller's family received an email and photograph over the weekend from her captors that enabled American intelligence to determine that she had been killed, US officials said.

Islamic State said on Friday that Mueller, 26, was killed when Jordanian fighter jets bombed a building where she was being held outside Raqqa, a stronghold in Syria of the Islamist militant group. Jordan and U.S. officials have expressed doubt about Islamic State's account of her death.

Mueller, from Prescott, Arizona, was thought to have been the last American held hostage by Islamic State, an al Qaeda offshoot that controls parts of Syria and Iraq. But on Tuesday, White House spokesman Josh Earnest said, ‘We are aware of other American hostages being held in the region.’ He did not identify them or provide details.

Among hostages thought to be held by Islamic State is British photojournalist John Cantlie, captured in northern Syria in November 2012.

American journalist Austin Tice disappeared in Damascus in August 2012, and last week his family renewed its campaign to draw attention to his case. It is unclear who is holding Tice or if he is still alive.

Mueller's family said in a statement they were ‘heartbroken’ to learn of her death, and released a letter she wrote in 2014 while in captivity.

She was captured in August 2013 while leaving a hospital in Aleppo in northern Syrian. Mueller had previously worked in Turkey providing humanitarian assistance to refugees from Syria's civil war.

In recent months, Islamic State has beheaded three Americans, two Britons, and two Japanese hostages, most of whom were aid workers or journalists.

US seeks justice

‘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,’ Obama said in a statement issued by the White House.

‘ISIL is a hateful and abhorrent terrorist group whose actions stand in stark contrast to the spirit of people like Kayla,’ Obama said, using an acronym for the group.

The White House said Obama called Mueller's parents to offer condolences.