peter kassig
A video posted online Sunday showed the murder of U.S. hostage Peter Kassig by ISIS militants. Handout

Update as of 06:28 a.m. EST: Abdul-Rahman Kassig's family called for him to be "remembered for his work" and the "love he shared," according to the BBC.

In addition, the video in question did not show the beheading of the man that it claims is Kassig. It did show a masked man standing with a decapitated head covered in blood lying at his feet. Speaking in English in a British accent, the man says: "This is Peter Edward Kassig a U.S. citizen," according to Reuters.

Update as of 0f 06:05 a.m. EST: U.S. National Security Council spokesperson Bernadette Meehan said in a statement: "We are aware of a video that claims to show the murder of U.S. citizen Peter Kassig... The intelligence community is working as quickly as possible to determine its authenticity. 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," according to a report from NBC News.

British Prime Minister David Cameron tweeted "I'm horrified by the cold blooded murder of Abdul-Rahman Kassig. ISIL have again shown their depravity. My thoughts are with his family."

Original story below:

A video posted online Sunday purports to show members of the Islamic State Group, or ISIS, beheading Peter Kassig, a U.S. aid worker, according to a report from Sky News.

The authenticity of the video has yet to be confirmed.

Sky also reported that the video appears to show 'Jihadi John', a militant responsible for previous hostage beheadings, who was believed injured after a recent U.S. airstrike targeted senior ISIS members.

The clip is over fifteen minutes long, and shows up to a dozen other men being beheaded, who are identified by the masked man in the video as Syrian officers and pilots from Bashar al-Assad's forces, according to Al Jazeera. He also identifies Kassig by name

Kassig, 26, is a former U.S. Army Ranger from Indiana who founded a group to help Syrian refugees called Special Emergency Response and Assistance, or SERA, after serving with U.S. forces in Iraq in 2007.

Kassig was detained in Syria in Oct. 2013. During his captivity, he converted to Islam, and took the first name Abdul-Rahman.

This is a breaking story. Check back for further updates.