SEOUL (AFP) - North Korean leader Kim Jong Un has hailed the recent test of a submarine-launched ballistic missile (SLBM) - which was viewed with wary scepticism outside the reclusive state - as a miraculous leap forward.

The North's official KCNA news agency reported on Tuesday that Mr Kim hosted a gathering of the scientists and technicians who were behind the test that was announced with great fanfare earlier this month.

It said Mr Kim congratulated them for producing an "eye-opening miracle" and a "historical event" that had left the country with a powerful strategic weapon.

A fully developed SLBM capability would take the North Korean nuclear threat to a new level, allowing deployment far beyond the Korean Peninsula and the potential to retaliate in the event of a nuclear attack.

But experts have questioned the authenticity of the recent test, saying photos of the launch had been digitally manipulated and suggesting the missile was probably fired from a sunken platform rather than a submarine.

Although the North clearly is set on developing a working SLBM, the expert consensus was that it remains years from acquiring such a capability.

Whether the test was exaggerated or not, South Korea said Tuesday it had requested a United Nations investigation into whether it constituted a sanctions violation.

Current UN resolutions ban North Korea from developing or using ballistic missile technology.

"We've sent a letter to the UN Sanctions Committee," Foreign Ministry spokesman Noh Kwang Il told reporters.

North Korea has been ramping up its nuclear rhetoric of late, boasting last week of its ability to miniaturise a nuclear warhead to fit on a missile.