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.
"They marry ideology and a sophistication of strategic and tactical military prowess," Defence Secretary Chuck Hagel said of the "barbaric" militants.
More From This Section
"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."
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 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.