NEW DELHI: The last known US citizen being held captive by the Islamic State in Syria, aid worker Kayla Mueller, has been confirmed dead by US authorities. The Islamic State claimed that she was killed in a Jordanian air strike, but the Pentagon noted that there is "no doubt" the militants killed her.

"Let's not forget in whose hands this woman died. And let's not forget who's ultimately responsible for it," US Navy Rear Admiral John Kirby said, although adding that exactly how Mueller was killed still remained unknown.

Jordan too denied that Mueller had been killed in air strikes launched following the public execution of captured Jordanian pilot Moaz al-Kasasbeh -- who was burned alive in a video produced by the militant group’s media arm.

The White House released a statement paying a tribute to Mueller. “Kayla dedicated her life to helping others in need at home and around the world. In Prescott, Arizona, she volunteered at a women’s shelter and worked at an HIV/AIDS clinic. She worked with humanitarian organizations in India, Israel, and the Palestinian territories, compelled by her desire to serve others. Eventually, her path took her to Turkey, where she helped provide comfort and support to Syrian refugees forced to flee their homes during the war. Kayla’s compassion and dedication to assisting those in need shows us that even amongst unconscionable evil, the essential decency of humanity can live on. Kayla represents what is best about America, and expressed her deep pride in the freedoms that we Americans enjoy, and that so many others strive for around the world,” the statement read.

The victim’s family also issued a heartfelt statement. “We are heartbroken to share that we’ve received confirmation that Kayla Jean Mueller has lost her life,” Mueller’s parents said in a statement.

The family also shared a touching letter written by Mueller whilst she was still in captivity last year.

A transcribed version of the letter is reproduced below:

Most of the Islamic State’s western hostages (and a handful of others including Kasasbeh from Jordan and Japanese hostage Kenji Goto) have been executed in videos circulated by the group. Shortly before news of Mueller’s death, a video showing captured Jordanian pilot -- Moaz al-Kasasbeh -- being burnt to death by the militants, was circulated -- prompting retaliatory strikes from Jordan, as mentioned.

Just shortly before that, a video showing the execution of Japanese hostage Kenji Goto -- whose fate was summarily tied to that of Kasasbeh -- was released. Previously, Goto had appeared in a video with another Japanese hostage, Goto made an appearance in a video with another Japanese hostage Haruna Yukawa, Yukawa was reportedly executed soon after.

The Islamic State has a number of foreign hostages, of which, in addition to Mueller, Kasasbeh, Goto and Yukawa, American aid worker Peter Kassig, US journalists James Foley and Steven Sotloff, and British aid workers David Haines and Alan Henning have been the most prominent executions. With the exception of Mueller and Yukawa, these executions were recorded -- the victims all dressed in orange jumpsuits -- and released by the Islamic State accompanied by messages directed to the victims’ countries for their role in bombing the militant group in Iraq and Syria.