US President Obama has confirmed the beheading of US aid worker Peter Kassig by Islamic State militants, after a video on social media purported to show his death. The militant group has further threatened to kill other US citizens “on your streets.”

“Today we offer our prayers and condolences to the parents and family of Abdul-Rahman Kassig, also known to us as Peter,” Obama told reporters on Air Force One during his homeward bound trip from Australia.

Obama went on to praise Kassig, labeling the act in which he was killed as one of “pure evil.”

Kassig, 26, was captured on October 1, 2013, by Islamic State (IS, formerly ISIS/ISIL) militants on his way to the city of Deir Ezzour in eastern Syria.

After conversion during captivity, Kassig took the name Abdul-Rahman.

The footage does not actually show the executioner, a British militant known as 'Jihadi John', kill Kassig. According to Reuters, John can be seen standing with a decapitated head at his feet.

"This is Peter Edward Kassig, a U.S. citizen," he says.

This comes despite earlier reports that the extremist, who has executed other UK and US hostages, was wounded in an airstrike on a top-level extremists’ meeting in Iraq last week.

The video allegedly showing Kassig’s death is part of 15 minutes of footage, which depicts John, alongside 18 other unmasked IS militants, beheading 20 men one by one. The victims have been identified as officers serving under the regime of Syrian President Bashar Assad.

The video shows the beheading of the Syrian army soldiers by foreign fighters among them a German convert. pic.twitter.com/VvSUCtCehy — Zaid Benjamin (@zaidbenjamin) November 16, 2014

In the video the extremists deliver warnings to the US, UK and other countries.

"To Obama, the dog of Rome, today we are slaughtering the soldiers of Bashar and tomorrow we will be slaughtering your soldiers," Reuters cites John as saying, predicting Washington would send more troops to the region to battle IS.

"And with Allah's permission ... the Islamic State will soon ... begin to slaughter your people in your streets."

Kassig first appeared on camera in the video showing the beheading of UK aid worker Alan Henning, with whom he shared a prison cell. After Jihadi John killed Henning, he also threatened Kassig would be next.

BREAKING NEWS: Islamic State Video shows beheading of UK hostage Alan Henning, warns US hostage Peter Kassig is next pic.twitter.com/46bNKjYcRW — Lisa Daftari (@LisaDaftari) October 3, 2014

The video was allegedly made in the Syrian town of Dabiq, in the country’s Aleppo province. The town holds propaganda value for IS, as of Dabiq is mentioned as a place where some events are expected to unfold during the Malahim or Armageddon. Dabiq is also the name given to IS' official propaganda magazine.

The footage has not been independently verified.

US authorities are “working as quickly as possible to determine its [the video] authenticity," spokeswoman Bernadette Meehan from the US National Security Council said in a statement.

"If confirmed, we are appalled by the brutal murder of an innocent American aid worker and we express our deepest condolences to his family and friends," she added.

The UK Foreign Office also said it is examining the video’s authenticity.

"We are aware of a further video and are analyzing its contents. If true, this is a further disgusting murder," the spokeswoman told Reuters.

UK PM David Cameron said he is “horrified” by the murder of Kassig.

"I'm horrified by the cold blooded murder of Abdul-Rahman Kassig. ISIL have again shown their depravity. My thoughts are with his family," he said.

READ MORE:ISIS executioner Jihadi John wounded in airstrike - reports

Kassig had served in Iraq in 2007 before becoming a medical technician. In May 2012, he travelled to Beirut, Lebanon where he volunteered in hospitals. He then helped in Palestinian refugee camps, “offering his services as a trauma medic to Syrian refugees wounded in the fighting in Syria.”

He founded his own aid organization, SERA (Special Emergency Response and Assistance), a non-governmental group intended to provide relief to Syrian refugees.

A Letter from Abdul-Rahman Kassig's Mother to IS Caliph Al Baghdadi @BintelSaifpic.twitter.com/NY4GTTiWWC — Paula Kassig (@PaulaKassig) October 8, 2014

His parents, Ed and Paula Kassig from Indiana, had been appealing for their son’s release. They tried to reach the IS leader by writing messages on Twitter.

“We have tried to contact you directly to plea for the life of our only son, Abdul Rahman Kassig, and have not received any response. Please tell us what more we can do so that Abdul Rahman may continue to serve and live his life in accordance with the teachings of Islam,” she wrote on Twitter.

The Kassig family released a letter from their son written on October 13, which was delivered to them by a hostage, who had been released. In the letter Kassig thanks his parents “for everything they have both done” for him. He describes his captivity, saying that he and the other hostages “share dreams and stories of home and loved ones.”

“I am obviously pretty scared to die but the hardest part is not knowing, wondering, hoping, and wondering if I should even hope at all,” he wrote. “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.”