The sophistication, wealth and military might of the Islamic State group represents a major threat to the United States that may surpass that once posed by al-Qaeda, US military leaders have said.
"They are an imminent threat to every interest we have, whether it's in Iraq or anywhere else," Defence Secretary Chuck Hagel told reporters at the Pentagon.
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.
Hagel's 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 several days after the group posted a video on social media showing one of its fighters beheading an American journalist who had been kidnapped in Syria.
Asked if the group posed a threat to the United States comparable to that of the attacks of 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."
Hagel spoke as the United States continued attacking Islamic State targets in Iraq. In the past two weeks, US drones and fighter jets have conducted 89 airstrikes against the group's 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. Obama ended the war in Iraq that killed thousands of American soldiers and consumed US foreign policy for nearly a decade,
Even after the gruesome killing of US journalist James Foley, Obama is seen as unlikely to deepen his near-term military involvement in either Iraq or Syria as he seeks to avoid becoming embroiled in another messy Middle Eastern conflict, Reuters news agency reported.
But US officials say they have not ruled out escalating military action against the Islamic State group, which has increased its overt threats against the United States since the air campaign in Iraq began.
"We haven't made a decision to take additional actions at this time, but we truly don't rule out additional action against ISIL if it becomes warranted," Ben Rhodes, a senior Obama aide, told National Public Radio earlier on Thursday, using another name for the group.
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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.
Dempsey suggested the group would remain a danger until it could no longer count on safe havens in areas of Syria under rebel control.
"This is an organisation that has an apocalyptic, end-of-days strategic vision and which will eventually have to be defeated," Dempsey said.
"To your question, can they be defeated without addressing that part of their organisation which resides in Syria? The answer is no. That will have to be addressed on both sides of what is essentially at this point a non-existent border."