With unusual choice of language, US Secretary of State wades into Islamic theology, claims Islamic State not true Muslims but 'apostates.'

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 terror 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.

"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."

Some Muslim legal scholars consider the proper punishment for turning one's back on the faith to be death and several majority Islamic countries execute convicted apostates.

ISIS claims to have founded a "caliphate" based on its interpretation of Islamic sharia law and itself often brands its Muslim enemies apostates.

Kerry was in Rome on Tuesday for a meeting of the 23 nations at the core of the US-led coalition fighting ISIS in Iraq and Syria and supporting local forces.

The end of a news conference by Kerry and Italy's foreign minister Paolo Gentiloni was briefly disrupted by protesters alleging US policy had caused the jihadists' rise.

AFP contributed this report.