'Islamic State threat one of the greatest'

The sophistication, wealth and military might of Islamic State militants represented a major threat to the US, surpassing that once posed by al-Qaeda, the US has said.

"They are an imminent threat to every interest we have, whether it's in Iraq or anywhere else," US Defence Secretary Chuck Hagel told reporters at the Pentagon. His assessment of Islamic State, which gained strength during Syria's civil war and swept into northern Iraq earlier this summer, sounded a note of alarm days after the terrorist group posted a video on social media showing one of its fighters beheading US journalist James Foley, who was kidnapped in Syria.

Asked if the hard-line Sunni Muslim organisation posed a threat to the US comparable to that of the attacks on September 11, 2001, Hagel said it was "as sophisticated and well-funded as any group we have seen… 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."

His statements come amid the US continuing its attacks on Islamic State targets in Iraq. In the past two weeks, US drones and fighter jets have conducted 89 airstrikes against militant targets in northern Iraq. So far, US President Barack Obama has sought to limit his renewed military campaign in Iraq to protecting American diplomats and civilians under direct threat.

It is unlikely Obama will deepen his near-term military involvement in either Iraq or Syria, as he seeks to avoid becoming embroiled in another conflict in West Asia. However, US officials say they aren't ruling out escalating military action against Islamic State, which has increased its overt threats against the US since the air campaign in Iraq began.

"We haven't decided to take additional action at this time, but we don't rule out additional action against Islamic State if it is warranted," Ben Rhodes, a senior Obama aide, told National Public Radio.

General Martin Dempsey, chairman of the US joint chiefs of staff, said officials were worried about the possibility that European or US nationals, radicalised after fighting in Iraq or Syria, would return to their home countries.

He suggested Islamic State would remain a danger until it could no longer count on safe havens in areas of Syria under militant control. "This is an organisation that has an apocalyptic, end-of-days strategic vision and which will eventually have to be defeated," he said.