US spy chief James Clapper on Tuesday warned that America faced a growing threat of a crippling cyber attack and voiced alarm over North Korea's recent "belligerent" rhetoric. In an annual assessment of global threats, the national intelligence director warned of an array of dangers around the world, from Pyongyang's bellicose stance to food shortages driven by extreme weather, but placed a particular emphasis on the threat posed by potential cyber attacks. The United States faces a mounting danger from digital assaults on power grids and other infrastructure.