The parents of a US woman held by Islamic State militants have been notified of her death, President Barack Obama has confirmed.
Carl and Marsha Mueller, the parents of Kayla Jean Mueller, released a statement saying they have been told she has died.
The White House also issued a statement confirming her death.
Islamic State said on Friday that the 26-year-old, from Prescott, Arizona, died in a Jordanian air strike.
The government of Jordan dismissed the statement as propaganda. US officials have said they had seen no evidence to corroborate the report.
Ms Mueller was the only known remaining US hostage held by IS. She was taken in August 2013 while leaving a hospital in Syria.
The US aid worker is believed to be the fourth American to die while being held by IS militants.
Three others – journalists James Foley and Steven Sotloff and aid worker Peter Kassig – were beheaded by the group.
The White House said Ms Mueller's family received a private message from her captors over the weekend and the information was authenticated by the US intelligence community.
It is not yet clear how and when she died.
Her parents said in a statement: “We are heartbroken to share that we’ve received confirmation that Kayla Jean Mueller has lost her life.
“Kayla was a compassionate and devoted humanitarian. She dedicated the whole of her young life to helping those in need of freedom, justice and peace.”
Mr Obama said Ms Mueller, who assisted humanitarian organisations working with Syrian refugees, “epitomised all that is good in our world”.
“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,” the president said.
Another American, journalist Austin Tice, disappeared in August 2012 while covering Syria’s civil war. It is not clear what entity is holding him, but it is not believed to be IS or the Syrian government, his family has said.
Jordan, which has launched a barrage of strikes on IS in recent days in retaliation for the killing of one of its pilots, disputed the claim that Ms Mueller died in a Jordanian air strike, and the White House said at the time that it had seen no evidence to corroborate the IS claims.
Ms Mueller had been working in Turkey assisting Syrian refugees, according to a 2013 article in the Daily Courier, her home town newspaper. She told the paper she was drawn to help with the situation in Syria.
“For as long as I live, I will not let this suffering be normal,” she said. “It’s important to stop and realise what we have, why we have it and how privileged we are. And from that place, start caring and get a lot done.”
According to the newspaper, she had been working with the humanitarian aid agency Support to Life, as well as a local organisation that helped female Syrian refugees develop skills.
A 2007 article about Ms Mueller from the same newspaper said she was a student at Northern Arizona University and was active in the Save Darfur Coalition. A statement from the office of Senator John McCain said she graduated in 2009 and had worked to help people in need in India, Israel, the Palestinian territories and in Arizona.