US president Barack Obama has said the beheading of Peter Kassig by the Islamic State group in Syria is “an act of pure evil”.

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.

Islamic State militants have beheaded another American hostage, Peter Kassig, issuing a video claiming the killing yesterday and warning the US they would kill other US citizens "on your streets."

US aid worker Peter Kassig has been beheaded by the Islamic State group, it has been confirmed.

The parents of a US aid worker held captive by the Islamic State group have revealed he told them in a letter he was afraid to die, but that he had converted to Islam voluntarily and was at peace with his beliefs.