The US has said a video circulated online showing members of the Islamic State of Iraq and the Levant (ISIL) group beheading several Syrian soldiers and US aid worker Peter Kassig is authentic.

The footage, released and confirmed by Washington on Sunday, showed the beheading of at least 12 people whom ISIL said were pilots and officers in Syrian President Bashar al-Assad's military.

Peter Kassig, 26, converted to Islam during captivity and changed his name to Abdul-Rahman [AP] Kassig, a 26-year-old from the state of Indiana, converted to Islam and changed his name to Abdul-Rahman during his captivity. US President Barack Obama confirmed the death of Kassig, calling his beheading "pure evil." "Abdul-Rahman was taken from us in an act of pure evil by a terrorist group that the world rightly associates with inhumanity," Obama said in a statement released aboard Air Force One as he flew home from a G20 summit in Australia. Before beheading one of the men in the video, a masked man says: "To Obama, the dog of Rome, today we are slaughtering the soldiers of Bashar and tomorrow we are slaughtering your soldiers. "And with god's permission we will break this final and last crusade."

The video also showed a bloodied head on the ground, whom ISIL said was that of Kassig.

"This is Peter Edward Kassig, a US citizen of your country. Peter, who fought against Muslims in Iraq, was serving as a soldier in the American army, does not have much to say," a masked man standing near the head said.

"His previous cellmates have already spoken on his behalf."

Al Jazeera's Imran Khan, reporting from Baghdad, said: "Peter had converted to Islam. He is a Muslim who died in the hands of Muslims. So it is a pretty controversial statement coming from ISIL."

Parents 'heartbroken'

Kassig's parents wrote on Twitter that they were "heartbroken" to learn that their son "has lost his life as a result of his love for the Syrian people and his desire to ease their suffering".

"Fed by a strong desire to use his life to save the lives of others, Abdul-Rahman was drawn to the camps that are filled with displaced families and to understaffed hospitals inside Syria," Ed and Paula Kassig said in their statement posted on Twitter.

"We know he found his home amongst the Syrian people, and the hurt when they were hurting."

Kassig founded an aid group through which he trained about 150 civilians to provide medical aid to people in Syria. His group also gave food, cooking supplies, clothing and medicine to the needy.

Also writing on Twitter, British Prime Minister David Cameron said: "I'm horrified by the cold-blooded murder of Abdul-Rahman Kassig.

"ISIL have again shown their depravity. My thoughts are with his family."

Two US journalists and two British aid workers had already been beheaded by ISIL in what the group says are retaliatory killings for the US-led air campaign against its fighters in Iraq and Syria.

Later on Sunday, the Syrian Observatory for Human Rights said ISIL had also executed al least 13 men in Syria's al-Raqqa city, who were arrested earlier this month after they were charged with collaborating with the Syrian regime.

The London-based rights group said that according to local sources the Syrian nationals were arrested about a month ago, adding that ISIL had said it had found individual weapons and wireless devices in their possession.

ISIL has executed numerous Iraqis and Syrians. In October, the group publicly executed Iraqi news cameraman Raad al-Azzawi and three civilians in northern Iraq, according to the journalist's relatives.