The United States said the Islamic State is the most dangerous group it has faced in recent years, and warned that the Middle East faces a long-term battle to defeat it.
Pentagon chiefs 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.
Speaking after the US military revealed it had already carried out a failed hostage rescue mission inside Syria, and against the backdrop of new air strikes in Iraq, they warned IS poses a considerable threat.
"They marry ideology and a sophistication of strategic and tactical military prowess," Defense 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."
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 organization 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."
Washington has launched air strikes against IS positions and boosted arms supplies to Kurdish forces, but still wants Baghdad's Shiite-led government to open its arms to Sunni moderates.
The warning came after IS released a video this week showing a militant with a British accent beheading American journalist James Foley, and threatening a second US hostage.
The murder has stoked fears in Britain and beyond 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 are about 12,000 foreign fighters from at least 50 countries in Syria.
A US official estimated that of those, more than 100 US citizens have tried to travel or actually traveled to Syria to join radical groups like the IS.
- Failed US rescue -
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.
Officials confirmed that in recent months, US special forces carried out a raid inside Syria to try to rescue people held hostage by IS militants, reportedly including Foley.
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 ($132 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.
State Department deputy spokeswoman Marie 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.
Pope Francis called Foley's relatives to express his condolences, a Catholic priest close to the family said on Twitter.
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.
Tunisia condemned the militants' "savage crimes," saying the group was a threat to all states in the region.
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.
Source: AFP