"We are heartbroken to learn that our son, Abdul-Rahman Peter Kassig, has lost his life as a result of his love for the Syrian people and his desire to ease their suffering," Ed and Paula Kassig said in a statement posted on Twitter.

A video released by IS militants showed the beheading of Kassig, who took the name Abdul-Rahman after converting to Islam, and that of 18 men described as Syrian military personnel.

His death was confirmed by President Barack Obama.

The 26-year-old was captured last year and was threatened in an October 3 video showing the beheading of British aid worker Alan Henning.

"Fed by a strong desire to use his life to save the lives of others, Abdul-Rahman was drawn to the camps that are filled with displaced families and to understaffed hospitals inside Syria," his parents wrote.

"We know he found his home amongst the Syrian people, and the hurt when they were hurting."

Kassig founded an aid group through which he trained some 150 civilians to provide medical aid to people in Syria. His group also gave food, cooking supplies, clothing and medicine to the needy.

"We are incredibly proud of our son for living his life according to his humanitarian calling," the Kassigs wrote, using the Twitter handle @kassigfamily.

"We remain eternally grateful for the many, many words of support and prayers from all over the world on our son's behalf."

They said they were also "heartbroken" for the families of other Western captives beheaded by IS – American journalists James Foley and Steven Sotloff, and British aid workers David Haines and Henning.

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