US Secretary of State John Kerry today described the feared Islamic State group as neither "a state nor truly Islamic", adding that America takes seriously the "unacceptable threat" posed by the terrorist organisation.

"The group calling itself the Islamic State is, in fact, neither a state nor truly Islamic. It is an adversary without a uniform, without any support by any government, and offering nothing in terms of coherent social or political programme," Kerry told a Washington audience.

"But it is a foe we take very seriously, in part because the dysfunction of some governments in the region has enabled these killers to seize control of more land and more resources than Al-Qaida ever had on the best day of its existence," he said.

The top US diplomat said the brutal militant outfit "has stolen vast quantities of weapons and money. It is attempting to recruit the fanatical and misguided in dozens of countries ... In the process, it has become a threat to America's core interests."