"I'm horrified by the cold blooded murder of Abdul-Rahman Kassig. ISIL [the Islamic State of Iraq and the Levant] have again shown their depravity. My thoughts are with his family," Cameron wrote on his Facebook account Sunday.
The IS issued a video on Sunday in which it purportedly killed an US aid worker Peter Kassig.
The video has not been verified as being genuine yet.
Sky News quoted US National Security Council Spokesperson Bernadette Meehan as saying that the White House would be "appalled by the brutal murder of an innocent American" should the video turn out to be authentic. According to the broadcaster, the United States is currently verifying the authenticity of the video.
The Islamic State, also known as the Islamic State of Iraq and Greater Syria (ISIS) or the Islamic State of Iraq and the Levant (ISIL), has been rebelling against the Syrian government since 2012 before it extended its fight to northern and western Iraq in June 2014 and declared a caliphate on the territories under its control.
The military advancement of the jihadist group has since started to raise serious concern, with the Unites States initiating an airstrike coalition in partnership with a number of countries to crack down on the militants. France, the United Kingdom and Australia are among the coalition countries that have launched airstrikes against the positions of the jihadists.
Before the Sunday beheading, the Islamic State had kidnapped and decapitated several other UK and US citizens in the past months.