The sophistication, wealth and military might of Islamic State (IS) militants pose a threat that may surpass that of al Qaeda, Pentagon leaders have warned.

US Defence Secretary Chuck Hagel said IS was "as sophisticated and well-funded as any group we have seen" after being asked if they posed a threat comparable to that of the September 11 2001 attackers.

"They are beyond just a terrorist group. They marry ideology, a sophistication of ... military prowess. They are tremendously well-funded. This is beyond anything we've seen," he said.

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General Martin Dempsey, chairman of the US Joint Chiefs of Staff, said Jihadists returning home from Syria and Iraq could pose a threat to Western nations.