North Korea's Kim Jong-Un hails 'successful' submarine missile test

North Korean leader Kim Jong-Un hailed a submarine-launched ballistic missile (SLBM) test as an "eye-opening success", state media said Sunday, declaring Pyongyang has the ability to strike Seoul and the US whenever it pleases.

Saturday's missile launch came amid growing concern that Pyongyang is preparing a fifth nuclear test.

But it was followed just hours later by a North Korean offer to impose a unilateral moratorium on nuclear testing if the United States suspends annual military exercises with South Korea.

The US and Britain denounced the SLBM test as a violation of UN Security Council resolutions and called on the North to refrain from further moves that could destabilise the region.

South Korea's defence ministry said the missile, fired from a submarine in the Sea of Japan, flew around 30 kilometres.

The North's state-run KCNA news agency said the test, personally monitored by Kim, confirmed the reliability of the country's underwater launching system.

Still images broadcast on state television showed Kim on the deck of the submarine before watching the test through binoculars from shore and meeting the crew and scientists afterwards.

North Korea has been pushing to acquire an SLBM capability that would take its nuclear strike threat to a new level, allowing deployment far beyond the Korean peninsula and the potential to retaliate in the event of a nuclear attack.

The isolated country has conducted a number of what it says were successful SLBM tests, but experts had previously questioned the claims, suggesting Pyongyang had gone little further than a "pop-up" test from a submerged platform.

The UN Security Council responded by slapping its strongest sanctions to date on Pyongyang.