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.

The submarine could significantly escalate security concerns on the Korean peninsula and in the Taiwan Strait, experts say North Korea's construction of a nuclear-powered submarine could significantly escalate security concerns on the peninsula and in the Taiwan Strait,

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.

North Korean leader Kim Jong-un inspected a project to build a nuclear-powered submarine, warning that his country's maritime defense capability will be "fully" projected "in any necessary waters without limitation.

Last week's accidental shelling of a South Korean village occurred less than 20 miles from the border of the nuclear-armed North.