Seoul is surrounded by three nuclear powers -- China, Russia, and North Korea, which invaded its neighbour in 1950 -- while it and neighbour Japan, both of them US allies but with difficult relations between them, rely on Washington's nuclear umbrella.

Pyongyang last week tested what it said was a submarine-launched ballistic missile, although the US said it seemed to have been fired from a "sea-based platform".

A proven submarine-based missile capability would take the North's arsenal to a new level, allowing deployment far beyond the Korean peninsula and a second-strike capability in the event of an attack on its military bases.

In a report submitted to parliament the South Korean Navy said it had set up a task force headed by a commander-level official to examine procuring nuclear-powered submarines in the long term, the South's Yonhap news agency reported.

It appears to be the first time Seoul has publicly spoken of developing nuclear submarines, which were not mentioned in its most recent defence white paper. The document, released in January, said the South had 10 conventional diesel-powered submarines, compared to 70 for the North.

According to South Korean media, the navy said the issue was "a matter to be determined as a national policy in consultation with the Defence Ministry and the Joint Chief of Staff" -- with no mention of the US.

Seoul is in a security alliance with the United States but President Donald Trump has repeatedly demanded it pay more towards the cost of US troops while his current emollient approach to the North's leader Kim Jong Un has raised some eyebrows in the South.

South Korean Navy chief Admiral Sim Seung-seob told MPs that nuclear-powered submarines capable of longer underwater operations than conventional boats would be "most effective in finding and destroying North Korean submarines equipped with submarine-launched ballistic missiles", according to Yonhap.

The military has "recognised their necessity and effectiveness", he added.

The defence ministry declined to confirm the news to AFP but endorsed citing the report.

According to Seoul's 2018 defence white paper, the US has 14 nuclear submarines, just ahead of Russia on 13, with China possessing four.

South Korea has 600,000 troops facing the North's nearly 1.3 million strong-military, the document says, while analysts estimate Pyongyang has between 20 to 60 nuclear warheads.