North Korea on Monday vowed to "steadily update and strengthen" its nuclear arsenal, rejecting a joint statement by the Group of Seven Foreign Ministers calling for Pyongyang to give up its nuclear ambitions.

North Korea has unveiled for the first time what it says is a nuclear-powered submarine under construction, a weapons system that can pose a major security threat to South Korea and the US.

North Korea has conducted a slew of underwater-launched ballistic missile tests since 2016, all from the same 2,000-tonne submarine with a single launch tube. This particular submarine has been described by experts as a test platform rather than an operational submarine in active service.