Islamic State militants are the most dangerous threat the US has faced in recent years, Washington has warned.

Defence Secretary Chuck Hagel said US air strikes had helped to break the Islamist advance in Iraq, but the militants could be expected to regroup.

America's top general Martin Dempsey stressed that IS could not be defeated without attacking their base in Syria.

The warnings come after IS posted a video showing the beheading of US journalist James Foley.

The US has now begun a formal criminal investigation into Mr 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 tried but failed earlier this summer to rescue Mr Foley and other US hostages held in Syria.

The militants had also reportedly wanted a $132m (£80m) ransom for his release.