North Korea on Saturday tested what appeared to be a submarine-launched ballistic missile in the Sea of Japan, the South Korean defence ministry said.

"North Korea launched a projectile which was believed to be a submarine-launched ballistic missile (SLBM) around 6:30 pm (0930 GMT) in the East Sea (Sea of Japan) near the northeastern port of Sinpo", a defence ministry spokesman said.

"We are keeping close tabs on the North Korean military and maintaining a full defence posture", he said.

It was not immediately known whether the launch was a success, he added.

The SLBM's engine ignited after it was ejected from a 2,000-tonne Sinpo-class submarine but was airborne for "a couple of minutes," South Korea's Yonhap News Agency said citing a government source.

North Korea has been pushing to acquire 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.

It has conducted a number of what it says were successful SLBM tests, but experts question the claim, suggesting Pyongyang had gone little further than a "pop-up" test from a submerged platform.

The test-firing comes as North Korea gears up for a rare and much-hyped ruling party congress early next month, at which leader Kim Jong-Un is expected to take credit for pushing the country's nuclear weapons programme to new heights.

Numerous analysts have suggested the regime might carry out a fifth nuclear test as a display of defiance and strength just before the congress opens.

Tension has been high on the divided Korean peninsula since Pyongyang's fourth nuclear test in January and rocket launch a month later that was widely seen as a disguised ballistic missile test.

The UN Security Council responded by imposing its strongest sanctions to date over the North's nuclear weapons programme.

Pyongyang has responded by staging a series of short- and mid-range missile tests and claiming a series of significant technical breakthroughs in its nuclear strike capability.

It claimed it had miniaturised a nuclear warhead to fit on a missile and successfully tested an engine designed for an inter-continental ballistic missile that could reach the US mainland.

While some experts say the claims are exaggerated, most acknowledge that the North's nuclear and ballistic missile programmes have made significant strides.