With an unusual choice of language, US Secretary of State John Kerry waded into Islamic theological debate on Tuesday when he branded the "Islamic State" (ISIS) terrorist group "apostates."

The United States affords its citizens religious freedom and does not consider apostasy a crime, but Kerry chose the term to rubbish the jihadists' claims of piety, AFP reports.

"Daesh is in fact nothing more than a mixture of killers, of kidnappers, of criminals, of thugs, of adventurers, of smugglers and thieves," he declared using the Arabic acronym for ISIS. "And they are also above all apostates, people who have hijacked a great religion and lie about its real meaning and lie about its purpose and deceive people in order to fight for their purposes."