Iraqi forces battled militants northeast of Baghdad today, as Washington said a powerful jihadist group was "beyond anything" it has seen and that operations in Syria may also be needed.
Pentagon chiefs warned of the dangers of the Islamic State (IS), 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.
But despite the jihadists' threat to kill a second reporter if it does not halt air strikes, Washington confirmed it had again bombed the militants in northern Iraq.
Iraqi government troops and Kurdish forces launched an operation today aimed at retaking the Jalawla area of Diyala province, northeast of the capital, from militants who seized it on August 11, officers said.
The operation came after Pentagon chiefs said that defeating IS, which spearheaded an offensive that overran swathes of Iraq, will require efforts across the region, including in neighbouring Syria, where the jihadists also control large areas.
"They marry ideology and a sophistication of strategic and tactical military prowess," Defence Secretary Chuck Hagel said of 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 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."
He was referring to the alienation of many Sunni Muslims from the Shiite-led government in Iraq and the Alawite-dominated regime in Syria.
The US military said it had conducted 90 air strikes in Iraq since August 8, more than half of them in support of Kurdish forces near Iraq's largest dam on the Tigris river north of militant-held second city Mosul.
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 launchpad for a new round of global terror attacks.
The US State Department said it estimated there were about 12,000 foreign fighters from at least 50 countries in Syria.
Foley, a 40-year-old freelance journalist, was kidnapped in northern Syria in November 2012. His employer GlobalPost said his captors had demanded a USD 132 million ransom.
GlobalPost CEO Philip Balboni said his team had never taken the demand seriously, and State Department deputy spokeswoman Marie Harf insisted: "We do not pay ransoms.