The US aid worker beheaded by the Islamic State wrote a letter to his family before his death saying they could take solace in the fact he died trying to “alleviate suffering”.

A video appearing to show the beheading of 26-year-old Peter Kassig was released on social media networks on Sunday and has been confirmed as authentic by the US government.

• Ebola, beheading videos ‘staged’: Naomi Wolf
• UK Muslims lead outcry over IS beheading
• See Mr Kassig’s full letter to his family below

Mr Kassig was captured last year while providing medical aid to Syrians.

US President Barack Obama has confirmed the authenticity of the video and the American’s death.

In a letter to his family, the former US serviceman said he was not angry about his situation.

“If I do die, I figure that at least you and I can seek refuge and comfort in knowing that I went out as a result of trying to alleviate suffering and helping those in need,” Mr Kassig wrote.

Peter Kassig working in the Middle East. Photo: Twitter

“Just know I’m with you. Every stream, every lake, every field and river. In the woods, and in the hills, in the places you showed me. I love you.”

He previously served in the US military in the Middle East in 2012, before founding his own relief organisation and travelling to Lebanon as a humanitarian worker in 2012.

While in captivity, he allegedly converted to Islam and changed his name to Abdul-Rahman.

Mr Kassig’s family said his captors had made demands for his release, but they were unable to meet them.

Prime Minister Tony Abbott said the video was “utterly barbaric” if real.

Mr Kassig is the fifth hostage beheaded by the Islamic State in recent months, after the murder of two US reporters and two British aid workers.

Peter Kassig’s letter to family

Mr Kassig wrote this letter to his family before his death. Photo: Twitter