The parents of an American aid worker beheaded by Islamic State militants after his abduction in Syria asked for prayers for other captives in Syria and Iraq in a brief public statement at their Indianapolis church on Monday.

Abdul-Rahman Kassig, whose name was Peter before his conversion to Islam, was the fifth European or American captive killed by the militants. His severed head was seen in a video that was released on Sunday and also showed the beheadings of at least 14 men.

In a brief public comment at Epworth United Methodist Church in Indianapolis, Kassig's mother, Paula Kassig, read words written about her son by one of his former teachers.

"If a person can be both a realist and an idealist, then that's Peter," she said. "Peter has earned the right to be both... Peter's life is evidence that he's been right all along; one person can make a difference."

Kassig's father, Ed Kassig, said: "Please pray for Abdul-Rahman, or Pete if that's how you know him, at sunset this evening. Pray also for all people in Syria and Iraq around the world who are held against their will."

In early October, Islamic State had threatened that Kassig, 26, a medic and former US Army Ranger, would be the next American or European captive the group would behead after killing British aid worker Alan Henning.

The Kassigs repeatedly appealed to Islamic State to spare their son, who had begun converting to Islam before he was captured in Syria in October 2013. They said his conversion was a sincere process. The family plans a joint Muslim-Christian memorial service for their son, a family spokeswoman said.

Kassig, who briefly served in Iraq in 2007 in the US Army, returned to the Middle East in 2012 for a spring break trip while studying political science, his family said.

Moved by the suffering of Syrian refugees, Kassig went to Lebanon to volunteer as an emergency medical technician, then founded an aid organization to provide food and medical supplies to refugees and first aid training to civilians in Syria, where some 200,000 people have died and millions are displaced.

Islamic State, an extremist group once linked to al Qaeda, has seized swathes of territory in Iraq and Syria and is now the target of US-led air strikes.