US President Donald Trump is "mentally deranged" and will "pay dearly" for his threat to destroy North Korea, Kim Jong-Un said Friday, as his foreign minister hinted the regime may explode a hydrogen bomb over the Pacific Ocean.

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.

North Korea says it has tested a hydrogen bomb which it can mount on a missile, declaring "perfect success" in its biggest-ever nuclear detonation and presenting a potent challenge to President Donald Trump.

North Korea fires a ballistic missile into the sea, South Korean and US officials say.

North Korean leader Kim Jong-Un says his country has successfully miniaturised a thermo-nuclear warhead, as Pyongyang continues to talk up its nuclear strike capabilities amid rising military tensions on the Korean peninsula.

A US long-range heavy bomber flew over South Korea, the US military says, days after North Korea conducted its first alleged hydrogen bomb test in defiance of international sanctions.