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, but all launches were made from the same 2,000-tonne-class submarine which has a single launch tube. Many experts call it a test platform, rather than an operational submarine in active service.