“This underlines the deplorable depths to which these terrorists are prepared to go, savagely murdering a compassionate man,” Cameron said in a statement.
“These beheadings show once again what a depraved organisation this is,” he added, referring to the killings of Kassig and 12 others.
A video posted online Sunday showed that Abdul-Rahman Kassig, a 26-year-old US aid worker, was allegedly beheaded by militants of the IS, earlier known as Islamic State of Iraq and the Levant (ISIL).
Kassig, a former US Army ranger, was captured by the IS in October last year while traveling to eastern Syria.
He was working with charity groups in Syria and Lebanon as a humanitarian worker.
“Like others who have been murdered in cold blood by ISIL, Abdul-Rahman Kassig was a selfless humanitarian worker who had gone to the region to help care for those fleeing the Syrian conflict,” Xinhua quoted the prime minister as saying.
“My heart goes out tonight to Abdul-Rahman’s family,” he continued.
It was reported that Kassig had converted to Islam during his capture and changed his name to Abdul Rahman.
The IS has previously released videos showing the apparent beheadings of two US journalists, James Foley and Steven Sotloff, as well as two British aid workers, David Haines and Alan Henning.