Islamic State (IS) militants are the most dangerous threat America has faced in years, top US officials have warned.
Defence Secretary Chuck Hagel said US strikes had weakened IS in Iraq, but the group could be expected to regroup, reports BBC.
America's top general Martin Dempsey said IS fighters could not be defeated without attacking its base in Syria.
The conflict has fuelled sectarian tensions in Iraq. In the latest attack, dozens of people have been killed in an attack on a Sunni mosque.
Officials say that during Friday prayers a suicide bomber detonated explosives inside the mosque in the eastern province of Diyala, and gunmen fired on fleeing worshippers.
It is not clear which group carried out the attack. The area has seen recent fighting between Iraqi troops backed by Shia militias and IS, a Sunni jihadist group,
Speaking at a news conference on Thursday, Mr Hagel described IS as an imminent threat.
"They are beyond just a terrorist group. They marry ideology, a sophistication of strategic and tactical military prowess, they are tremendously well-funded. This is beyond anything that we have seen."
Meanwhile, Gen Dempsey, the chairman of the US Joint Chiefs of Staff, said IS was "an organisation that has an apocalyptic, end-of-days strategic vision and which will eventually have to be defeated".
"To your question, can they be defeated without addressing that part of their organisation which resides in Syria? The answer is no. That will have to be addressed on both sides of what is essentially at this point a non-existent border."
Neither Mr Hagel nor Gen Dempsey announced a change in the limited military campaign adopted by Barack Obama, and the US president is unlikely to deepen his involvement in Iraq or Syria, the BBC's Barbara Plett Usher in Washington reports.
But US officials did not rule out additional action against IS in Iraq or Syria, our correspondent adds.
Britain said it would not work with Syrian President Bashar al-Assad in the battle against IS, despite suggestions from a retired top general that it should do so.
The warnings come after IS beheaded US journalist James Foley. The US has now begun a criminal investigation into his death, with US Attorney General Eric Holder warning that the country has a "long memory".
It has emerged that a special US military mission tried but failed earlier this summer to rescue Mr Foley and other US hostages held in Syria.
In the UK, police and security services are trying to identify the jihadist who appeared in footage of Mr Foley's killing.
