The parents of an American aid worker captured last year while delivering relief supplies to refugees in Syria said they are "heartbroken" by his death at the hands of Islamic State militants.

But Ed and Paula Kassig of Indianapolis said they were "incredibly proud" of his compassion and humanitarian work.

They said their 26-year-old son Peter Kassig "lost his life as a result of his love for the Syrian people and his desire to ease their suffering.

"We will work every day to keep his legacy alive as best we can."

Mr Kassig was captured last year in eastern Syria while delivering relief supplies to refugees of Syria's civil war. The Indianapolis man, a former US Army Ranger who founded a relief organisation, converted to Islam while in captivity and took the first name Abdul-Rahman.

The White House confirmed Mr Kassig's death yesterday after the Islamic State (IS) group released a video showing that he had been beheaded. The video also showed the beheadings of about a dozen men identified as Syrian military officers and pilots.

President Barack Obama called Mr Kassig's killing "an act of pure evil" and said IS "revels in the slaughter of innocents, including Muslims, and is bent only on sowing death and destruction".

The Kassigs said in their statement that their hearts also go "out to the families of the Syrians who lost their lives, along with our son".

They said they also grieve "for the families of the other captives who did not make it home safely".

The Kassigs learned of their son's capture last year, but did not disclose his captivity while family and friends quietly worked to secure his release.

In October, their son appeared in another video released by IS that showed the beheading of a fellow aid worker, Britain's Alan Henning. The militants vowed that Mr Kassig would be next, leading his parents to plead publicly for mercy while stressing his humanitarian work and conversion to Islam.

Mr Kassig first went to the Middle East with the US Army, which he joined in 2006, according to his military records.

He ultimately served in the 75th Ranger Regiment, a special operations unit, and served in Iraq from April until July 2007 before being medically discharged as a private first class that September.

His desire to perform aid work in the region was kindled during a March 2012 spring break trip to Beirut while he was studying political science at Butler University.

Mr Kassig, a certified emergency medical technician, left school and returned two months later to Lebanon, where he worked as a medical assistant and humanitarian worker and treated people from all sides of the conflict in neighbouring Syria.

In September 2012, he founded Special Emergency Response and Assistance, or SERA, which suspended its relief work after his capture.

Mr Kassig, who grew up in a family with a long history of humanitarian work and teaching, said during a January 2013 interview with Time magazine that he travelled heavily throughout Lebanon to assess the needs of people there.

SERA, he said, focused on supplementing the work of larger organisations by delivering aid that could "do the most good for the most people over the longest period of time possible".

Mr Kassig's friends and family say he understood the risks of working in the region, but he felt called to help.

Burhan Agha, a 26-year-old Syrian, worked with Mr Kassig in the northern Lebanese city of Tripoli, delivering aid to Syrian refugees before Mr Kassig moved his operations to southern Turkey.

Speaking by phone from Switzerland, where he is seeking asylum, Mr Agha described his friend's killing as senseless.

"If I could apologise to each American, one by one, I would," he said while weeping.

"Because Peter died in Syria, while he was helping the Syrian people. And those who killed him claimed to have done it in the name of Islam. I am a Muslim, and from Syria, and he is considered a part of the Syrian revolution."