U.S. military involvement in Iraq will continue because the long-term threat from ISIS is more dangerous, even than that from al-Qaeda, Defense Secretary Chuck Hagel said Thursday.

In a briefing at the Pentagon a day after the U.S. revealed that it launched a failed mission in Syria this summer to rescue U.S. hostages being held by the Islamic State of Iraq and al-Sharm, Hagel said the U.S. is "very clear-eyed about the challenges ahead," stressing: "The U.S. military's involvement is not over."

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Asked to compare ISIS to al-Qaeda, Hagel said it was "beyond anything that we've seen," calling the group more than "just a terrorist group." "They marry an ideology with a sophistication of strategy and military prowess" that represent "a whole new dynamic and a new paradigm of threats to this country," he said.

Hagel strongly defended the mission in Syria to rescue journalist James Foley and other hostages earlier this summer, saying the Obama administration had plenty of intelligence to justify the operation. "This operation, by the way, was a flawless operation, but the hostages were not there," he said. Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, added that he believed that at some point the hostages definitely were at the location the U.S. raided.

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MIssion creep? Hagel: "The president's been very clear about mission creep" we are w/in war powers. "This is not about mission creep" #Iraq — Kevin Baron (@DefenseBaron) August 21, 2014

— M. Alex Johnson and Jim Miklaszewski