Islamic State 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. 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.