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.
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.