The Islamic State group released a graphic video Sunday in which a black-clad militant claims to have beheaded U.S. aid worker Peter Kassig, who was captured last year.

The militant was standing over a severed head, but it was not immediately possible to confirm that Kassig, 26, was pictured in the video. Family representatives were not immediately available for comment.

The video, which was posted on websites used by the group in the past, appeared to be the latest in a series of grisly messages to the U.S. warning of further brutality if it does not abandon its air campaign in Iraq and Syria.

"This is Peter Edward Kassig, a U.S. citizen, of your country; Peter who fought against the Muslims in Iraq, while serving as a soldier," the militant says near the end of the nearly 16-minute video. He speaks in an audible British accent despite his voice being distorted to make it more difficult to identify him.

The video also shows what appears to be the mass beheading of several Syrian soldiers captured by the group. The militants warn that U.S. soldiers will meet a similar fate.

Kassig, a U.S. Army Ranger, was providing medical aid to Syrians fleeing the country's civil war when he was captured inside Syria on Oct. 1, 2013. His friends say he converted to Islam in captivity and took the first name Abdul-Rahman.

The video identifies the militant's location as Dabiq, a small town in the northern Syrian province of Aleppo, near the Turkish border.

Previous videos have shown the beheading of two American journalists and two British aid workers. The latest video did not show the person identified as Kassig being beheaded.

Kassig formed the aid organization Special Emergency Response and Assistance, or SERA, in Turkey to provide aid and assistance to Syrian refugees. He began delivering food and medical supplies to Syrian refugee camps in 2012 and is also a trained medical assistant who provided trauma care to injured Syrian civilians and helped train 150 civilians in providing medical aid.

The militant Islamic State group has beheaded and shot dead hundreds of captives -- mainly Syrian and Iraqi soldiers -- during its sweep across the two countries, and has celebrated mass killings in a series of slickly produced but extremely graphic videos.

The group has declared an Islamic caliphate in the areas under its control in Syria and Iraq, which it governs according to a harsh version of Shariah law.

The U.S. began launching air strikes in Iraq and Syria earlier this year in a bid to halt the group's rapid advance and eventually degrade and destroy it.