Islamic State militants are the most dangerous threat the US has faced in recent years, warned Washington.
Defence Secretary Chuck Hagel said that the US air strikes had helped to break the militant’s advance in Iraq, but they could be expected to regroup. America's top general Martin Dempsey stressed that ISIS could not be defeated without attacking their base in Syria.
The warnings came after ISIS posted a video showing the beheading of US journalist James Foley. The US has begun a formal criminal investigation into Foley's death, with US Attorney General Eric Holder warning that the country has a "long memory".
It has emerged that a special US military mission failed to rescue Foley and other US hostages held in Syria. The militants had also reportedly wanted a $132m (£80m) ransom for his release.
Speaking at a news conference on Thursday, Hagel described ISIS as an imminent threat
US aircraft destroyed or damaged four ISIS vehicles and several bomb placements in strikes near the strategic Mosul Dam in northern Iraq on Thursday, the military said.
There have been a total of 90 air strikes across Iraq since operations began, the Pentagon said. Of those 90 strikes, 57 have been near the dam. The US said Iraqi troops and Kurdish fighters recaptured the dam with American assistance on Monday.
ISIS fighters have captured large swathes of Iraq killing many and leaving around an estimated 1.2 million people in Iraq alone, homeless.