Murdered US hostage James Foley told his parents of his life imprisoned with 17 other captives in a Syrian dungeon, in a moving smuggled letter.

The 40-year-old freelance reporter -- whose death was revealed last week in a video released by militants from the so-called "Islamic State" -- taught a fellow hostage to memorize his message.

The hostage in turn dictated the letter to Foley's parents John and Diane Foley after his release, leaving them with a heart-breaking memento of their son before his brutal end.

In the message, he tells of being imprisoned with a multinational group of IS hostages -- some but not all of them since released -- and of the camaraderie that kept them together through the ordeal.

The letter, as published on the family's Facebook page, recalls how memories of family life growing up in a middle-class New Hampshire family with four siblings kept Foley's spirits up.

It also touches on his faith. A devout Catholic, he says his prayer helps keep him close to his parents.

"I know you are thinking of me and praying for me. And I am so thankful. I feel you all especially when I pray. I pray for you to stay strong and to believe. I really feel I can touch you even in this darkness when I pray," it says.

"Eighteen of us have been held together in one cell, which has helped me. We have had each other to have endless long conversations about movies, trivia, sports," Foley says.

"I have had weak and strong days. We are so grateful when anyone is freed; but of course, yearn for our own freedom. We try to encourage each other and share strength. We are being fed better now and daily. We have tea, occasional coffee. I have regained most of my weight lost last year."

Foley send personal messages to his brothers and sisters and his grandmother, recalling vacations together and happier family times.

"Grammy, please take your medicine, take walks and keep dancing. I plan to take you out to Margarita's when I get home. Stay strong because I am going to need your help to reclaim my life."

The 40-year-old reporter, who had covered wars in Afghanistan, Libya and Syria and contributed to GlobalPost, Agence France-Presse and other outlets was seized by armed men in northern Syria in 2012.