The parents of slain US aid worker Peter Kassig said Monday they would try to "forgive" Islamic State jihadists who beheaded their son, calling for healing and prayer as they mourned their loss.

A video released Sunday by IS militants showed the beheading of 26-year-old Kassig, who took the name Abdul-Rahman after converting to Islam.

"Please allow our small family the time and privacy to mourn, cry and -- yes -- forgive and begin to heal," Peter's father Ed said in an emotional address outside his church.

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

Peter's mother Paula said while their world had been torn apart by the death of their son, they would focus on healing.

"Rather than letting the darkness overwhelm him he has chosen to believe in the good, in himself and in others... One person makes a difference," she said.

"Our hearts are battered. But they will mend. The world is broken, but it will be healed in the end."

Peter, a former US soldier, was captured last year and is the fifth Western hostage killed by IS in recent months, after two American journalists and two British aid workers were beheaded.

The undated video -- featuring a man who appears to be the same British-accented militant who beheaded previous Western hostages -- also broadcasts the murder of 18 men described as Syrian militants.

The British government has said it is looking into a newspaper report that the accented fighter, dubbed "Jihadi John," had been injured in a US-led air strike last week.

Kassig founded an aid group through which he trained some 150 civilians to provide medical aid to people in Syria. The Iraq war veteran had risked his life to give food, cooking supplies, clothing and medicine to the needy.

US President Barack Obama called Kassig's murder an act of "pure evil," while British Prime Minister David Cameron said he was "horrified" by the killing.

Kassig's beheading came as Washington prepares to double its military personnel in Iraq to up to 3,100 as part of the international campaign it heads against the IS group.

Despite daily air raids by the US-led coalition, the IS group has continued to advance in some areas and has yet to be driven back from key towns it seized over the past several months in Iraq and Syria.