The White House on Sunday called killing of U.S. aid worker Abdul-Rahman Kassig "an act of pure evil" as it offered its prayers and condolences to Kassig's family after the militant group ISIS announced his death in a 16-minute video.

The video features a masked militant with a British accent standing over a severed head. "This is Peter Edward Kassig, a U.S. citizen," the militant says, referring to him by the name Kassig used before he converted to Islam. Kassig, 26, of Indiana was working as a humanitarian aid worker in Syria when he was captured in October 2013.

"Abdul-Rahman was taken from us in an act of pure evil by a terrorist group that the world rightly associates with inhumanity, President Barack Obama said in a statement Sunday afternoon, adding that while ISIS "revels in the slaughter of innocents, including Muslims, and is bent only on sowing death and destruction, Abdul-Rahman was a humanitarian who worked to save the lives of Syrians injured and dispossessed by the Syrian conflict." British Prime Minister David Cameron said that the video illustrated the "depravity" of ISIS and that he was "horrified by the cold blooded murder of Abdul-Rahman Kassig."

Let our news meet your inbox. The news and stories that matters, delivered weekday mornings. This site is protected by recaptcha

The video also shows the beheadings of people who appear to be Syrian soldiers. The militant — who sounds similar to the man with a London accent featured in previous beheading videos — goes on to say Obama had lied about withdrawing troops from Iraq. Intelligence analysts SITE said the video was produced by ISIS' propaganda unit, the al-Furqan Media Foundation.

Kassig said in a letter to his parents in June that he was "pretty scared to die," urging them to "seek refuge and comfort" in the fact that he was providing humanitarian relief when he was captured.

NBC News terrorism analyst Evan Kohlmann of Flashpoint Global Partners said the video was also significant for what it didn't show: "Each previous beheading video has introduced a new Western hostage — and here we have none. This suggests that either the life of British national John Cantlie is perhaps now in danger, or that they are simply running out of hostages."

IN-DEPTH

SOCIAL

I'm horrified by the cold blooded murder of Abdul-Rahman Kassig. ISIL have again shown their depravity. My thoughts are with his family. — David Cameron (@David_Cameron) November 16, 2014

- Alastair Jamieson