Kayla Mueller, who died as a hostage of IS jihadists, took strength during captivity in her faith in God and the love of her family, she wrote in a letter relatives released on Tuesday.

"I have been shown in darkness, light + have learned that even in prison, one can be free," said a handwritten letter by Mueller which was smuggled out by fellow captives following their release by Islamic State jihadists.

The 26-year old humanitarian aid worker was driven by an unquenchable passion to help others, saying that service to others brought her closer to God.

In the letter released by her family after the White House confirmed her death, she said she managed to find glimmers of joy, even in captivity.

"I have come to see that there is good in every situation, sometimes we just have to look for it, Mueller wrote in the letter received by her relatives in early 2014.

Mueller described the place where she was being held as "a safe location," and said she was "completely unharmed + healthy (put on weight in fact)."

She also revealed times of doubt and darkness, but also resolve as she summoned the will to keep going.

"None of us could have known it would be this long but know I am also fighting from my side in the ways I am able + I have a lot of fight left inside of me," she wrote in her letter.

- Humanitarian service -

The idealistic native of Prescott, Arizona, was captured by IS fighters in August 2013, while leaving a Doctors Without Borders hospital in Aleppo, Syria.

Her humanitarian service over the years included volunteer work at a women's shelter in her hometown of Prescott, as well as work at an HIV/AIDS clinic.

At the time she was taken hostage, Mueller had been working along the Turkish-Syrian border, trying to help some of the thousands of refugees fleeing the Syrian civil war.

That conflict was but one of several where Mueller had hoped to make a difference.

Since her graduation from Northern Arizona University in 2009, she dedicated her life to helping those in need across the globe from India, to Israel, to the Palestinian territories.

Mueller was the last remaining American hostage known to be held by the jihadist group, which has made a grisly ritual of killing a succession of hostages, then posting the brutal executions on the Internet.

Mueller's relatives only last week revealed that she was in fact, being held by IS, after having requested to keep her captivity secret since her abduction.

After IS last week announced that Mueller had died as a casualty of coalition bombing, her family continued to cling to the hope that she somehow had escaped death.

But over the weekend they received a "private message" from IS that allowed US intelligence services confirm her death, the White House and the family said.

- Life with purpose -

The family said they are buoyed by the knowledge of how much good Mueller did during her short life.

"She lived with purpose, and we will work every day to honor her legacy," they said.

Eryn Street, a friend, later told reporters in her hometown of Prescott: "I'm not yet sure how to live in a world without Kayla, but I do know that we're all living in a better world because of her."

One of her aunts, Lori Lyon, read out a statement from her aunts and uncles, saying: "She had a quiet, calm presence. She was a free spirit, always standing up for those who were suffering and wanting to be their voice.

"Kayla's calling was to help those who were suffering, whether in her hometown of Prescott, or on the other side of the world," Lyon added.

President Barack Obama paid homage to Mueller's brief, purpose-driven existence.

"She epitomized all that is good in our world," the president said in a statement.