WASHINGTON - The United States said Thursday that the Islamic State was "beyond anything" it has seen, as US warplanes pressed on with airstrikes against it despite death threats against an American hostage.
Pentagon chiefs warned of the dangers of a slick, well-funded operation powered by an "apocalyptic end of days" ideology as the West reeled from the grisly execution of American journalist James Foley.
However despite threats to kill a second reporter if the US did not halt airstrikes against the Islamic State (IS), the US confirmed it had again bombed the militants in northern Iraq near the Mosul dam.
US military leaders said the jihadist group could be eradicated if local Sunni communities reject it and regional powers unite to fight it, but only if the battle is taken into Syria and not just Iraq.
Their warnings came after the US military revealed it had carried out a failed mission to resue American hostages inside Syria, reportedly including Foley.
"They marry ideology and a sophistication of strategic and tactical military prowess," Defence Secretary Chuch Hagel said about the "barbaric" militants.
"They are tremendously well funded. This is beyond anything we have seen."
General Martin Dempsey, chairman of the Joint Chiefs of Staff, said the group "has an apocalyptic end of days strategic vision that will eventually have to be defeated."
A 'very long contest'
Dempsey warned the jihadist vision of a wider Muslim caliphate could "fundamentally alter the face of the Middle East and create a security environment that would certainly threaten us in many ways."
"Can they be defeated without addressing that part of the organisation that resides in Syria? The answer is no," he said, when asked if the campaign against the group could go beyond Iraq.
He spoke of a "very long contest" that could not be won by US military prowess alone, but only with regional support and that of "the 20 million disenfranchised Sunnis that happen to reside between Damascus and Baghdad."
The US military said it has conducted 90 air strikes in Iraq since August 8, more than half of them in support of Iraqi government forces near Mosul dam.
The murder of Foley has stoked fears in the West that the territory the militants have seized in Syria and northern Iraq could become a launching pad for a new round of global terror attacks.
And as part of that worrying trend, the US State Department estimated that there were about 12,000 foreign fighters from at least 50 countries in Syria.
'We do not pay ransoms'
Foley, a 40 year-old freelance journalist, was kidnapped in northern Syria in November 2012. His employer GlobalPost said his captors had demanded a 100-million-euro (S$165.8 million) ransom.
GlobalPost CEO Philip Balboni said his team had never taken the demand seriously, and State Department deputy spokeswoman Marie Harf insisted bluntly that: "We do not pay ransoms."
The captors had also sent Foley's family a taunting and rambling email threatening to kill him.
GlobalPost released the text, which claims that "other governments" had accepted "cash transactions" for the release of hostages, and says that the militants had offered prisoner exchanges for Foley's freedom.
Harf said the United States estimates that IS militants have already received millions of dollars worth of ransoms so far this year alone. Most are believed to have been paid by European governments.
In the execution video, released online, a black-clad militant said that Foley was killed to avenge US air strikes against IS.
The man, speaking with a clear south London accent, then paraded a second US reporter, Steven Sotloff, before the camera and said he would die unless President Barack Obama changes course.
Sotloff is a freelance journalist for Time magazine who was captured on August 4, 2013.
US Attorney General Eric Holder said that the FBI already had opened a criminal probe into Foley's kidnap.
The scale of the threat from IS became clear in June when the group, then known as the Islamic State in Iraq and the Levant, declared the dawn of a caliphate and seized the Iraqi city of Mosul.
An Iraq offensive by IS jihadists, who are accused of multiple acts of summary execution, rape and other atrocities, has caused some 200,000 people, mostly members of the minority Christian and Yazidi communities.
Obama reacted this month by ordering US warplanes to counter threats to US personnel in the Kurdish regional capital Arbil and to civilian refugees from Iraqi religious minority groups.