Kayla Mueller, the young US aid worker who died while a captive of militants of the Islamic State group in Syria, was remembered by family and friends yesterday as a "free spirit" driven by her enormous empathy for human suffering.

Mueller, 26, was confirmed to have died under circumstances that remain unclear about 18 months after she was abducted while leaving a hospital in northern Syrian.

The White House said Mueller's family received a message from her captors containing information that was authenticated by US intelligence analysts who verified her death.

President Barack Obama also publicly acknowledged in an interview with the news website Buzzfeed that Mueller was among the hostages whom US commandos were sent to rescue but failed to find in an operation he ordered last year.

Outside the county courthouse in her hometown of Prescott, Arizona, two of Mueller's maternal aunts joined one of her childhood friends and a local pastor to deliver a tearful tribute to her.

"She had a quiet, calming presence. She was a free spirit, always standing up for those who were suffering and wanting to be their voice," Lori Lyon said of her niece in a statement.

"Kayla had such great empathy, and it's hard to find that in this world. It's really rare, and it was her greatest strength," said Eryn Street, who described Mueller as her closest friend and "kindred spirit".

Kathleen Day, head of the United Christian Ministry at Northern Arizona University, related accounts of how Kayla's upbeat mood prevailed even in captivity, as she tried to teach origami to her guards, and exercised in cramp quarters by doing headstands.

Near the courthouse just off the main town square stood a small, makeshift shrine adorned with flowers, a cross, a toy bear and a handwritten note: "God bless you Kayla." A large white sign with the words, "Pray for Kayla" stood beside it.

Prescott Mayor Marlin Kuykendall, a friend of the Muellers, said there had been talk of a memorial service, but that will take some time and be dictated by the wishes of her family.

Before her abduction, Mueller worked for a Turkish relief agency on the Syrian border, first traveling to that region in 2012. She previously had volunteered for schools and relief groups elsewhere in the Mideast and India.

"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," her parents, Marsha and Carl, said in a statement.

The text of an emotional letter she wrote during her confinement was also released. She assured readers that she was safe and unharmed, and had been treated with "the utmost respect + kindness".

The letter said: "If you could say I have 'suffered' at all throughout this whole experience it is only in knowing how much suffering I have put you all through; I will never ask you to forgive me as I do not deserve forgiveness. I remember mom always telling me that all in all in the end the only one you really have is God. I have come to a place in experience where, in every sense of the word, I have surrendered myself to our creator b/c literally there was no else....+ by God + by your prayers I have felt tenderly cradled in freefall.

"I have been shown in darkness, light + have learned that even in prison, one can be free. I am grateful. I have come to see that there is good in every situation, sometimes we just have to look for it. I pray each each day that if nothing else, you have felt a certain closeness + surrender to God as well + have formed a bond of love + support amongst one another... "

It concludes: "Please be patient, give your pain to God. I know you would want me to remain strong. That is exactly what I am doing. Do not fear for me, continue to pray as will I + by God's will we will be together soon."

Additional reporting by Reuters