North Korea says a nuclear war on the Korean Peninsula has become a matter of when, not if, as it continued to lash out at a massive joint military exercise between the United States and South Korea involving hundreds of advanced warplanes.

North Korea conducts the sixth nuclear test and declares itself in command of a working hydrogen bomb just hours after the explosion. Here are some key questions around the blast and the isolated state's nuclear programme.

British Prime Minister Theresa May, due in Tokyo is "outraged" by North Korea firing a ballistic missile over Japan, her Downing Street office says.