North Korea successfully conducted new tests of its long-range cruise missiles for submarines as it continues to ratchet up tensions with the West.

The Pulhwasal-3-31 missiles were reported to have accurately hit their targets, flying 1,500 km at a speed of 200 metres per second.

Pyongyang said the tests were conducted "as part of an overall plan to implement efforts to build the country’s defence capabilities".

South Korea said its military spotted the missiles being fired towards the Yellow Sea at about 4pm local time on Saturday.

“Detailed specifications are currently being closely analysed by the intelligence authorities of South Korea and the United States,” the Joint Chiefs of Staff said.

Pyongyang first tested its cruise missile last January, firing it from a ground launcher.

One month later in February they repeated the test, this time firing the Pulhwasal-3-31 from a submarine.

The Pulhwasal-3-31 is believed to be an adaptation of the Hwasal cruise missile, which is capable of carrying a nuclear warhead.

This is the second major missile test that Kim Jong un has carried out so far this year.

Earlier in January, the North Korean dictator fired off a new intermediate-range ballistic missile.

North Korea's official news agency, KCNA, said the missile flew about 1,500 kilometres and 12 times faster than the speed of sound.

Pyongyang appears to be benefiting from its military pact with Moscow, which has seen Kim send Putin both men and weapons for his war in Ukraine.

North Korea is reported to be receiving Russian military technology in exchange, allowing it to upgrade its submarine and missile capabilities.

The war in Ukraine allows Pyongyang to assess the effectiveness of its weapons on the battlefield.

Experts say this will help North Korea improve the quality of its own domestic arms industry and adapt future output to the realities of modern warfare.

In addition, North Korean soldiers are gaining first-hand experience of modern combat conditions.

All in all, this strengthens North Korea's army and military-industrial complex, making the country even more of a security threat to its neighbours and the world in general.