WASHINGTON (AFP) - The parents of United States aid worker Peter Kassig said on Monday they were "battered" after the death of their son, who was brutally beheaded by Islamic State of Iraq and Syria (ISIS) militants.

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

"Our hearts are battered. But they will mend. The world is broken, but it will be healed in the end," said Mr Kassig's mother, Paula.

His father urged the world to pray for Mr Kassig in the emotional message, and asked for privacy as they mourn the loss of their son.

"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," he said.

"And lastly, please allow our small family the time and privacy to mourn, cry and, yes, forgive and begin to heal."

The former US soldier was captured last year and is the fifth Western hostage killed by ISIS 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.

Mr 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.

US President Barack Obama called Mr Kassig's murder an act of "pure evil". His killing 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 ISIS.