North Korean leader Kim Jong Un has supervised test firings of cruise missiles designed to be launched from submarines and reviewed efforts to build a nuclear-powered submarine, state media said on Monday.

Mr Kim reiterated his goal of building a nuclear-armed navy to counter what he portrays as growing external threats.

The report came a day after South Korea’s military said it had detected North Korea firing multiple cruise missiles over waters near the eastern port of Sinpo, where the North has a major shipyard developing submarines.

It was the latest in a streak of weapon demonstrations by North Korea amid increasing tensions with the United States, South Korea and Japan.

North Korea’s official newspaper Rodong Sinmun published photos of what appeared to be at least two missiles fired separately. Both created grayish-white clouds as they broke the water surface and soared into the air at an angle of around 45 degrees, which possibly suggests they were fired from torpedo launch tubes.

State media said the missiles were Pulhwasal-3-31, a type of weapon first tested last week in land-based launches from North Korea’s western coast.

The reports implied two missiles were fired during the test but did not specify the vessel used for the launches. North Korea has fired missiles both from developmental, missile-firing submarines and underwater test platforms built on barges.

In recent years, North Korea has tested a variety of missiles designed to be fired from submarines as it pursues the ability to conduct nuclear strikes from underwater. In theory, such capacity would bolster its deterrent by ensuring a survivable capability to retaliate after absorbing a nuclear attack on land.

Missile-firing submarines would also add a maritime threat to the North’s growing collection of solid-fuel weapons fired from land vehicles that are designed to overwhelm missile defenses of South Korea, Japan and the United States.

The North’s official Korean Central News Agency (KCNA) said Mr Kim expressed satisfaction after the missiles accurately hit their sea targets during Sunday’s test.

He issued unspecified important tasks for “realising the nuclear weaponisation of the navy and expanding the sphere of operation,” which he described as crucial goals considering the “prevailing situation and future threats,” the report said.

KCNA said Mr Kim was also briefed on efforts to develop a nuclear-propelled submarine and other advanced naval vessels.

Nuclear-propelled submarines can quietly travel long distances and approach enemy shores to deliver strikes, which would bolster Mr Kim’s declared aim of building a nuclear arsenal that could viably threaten the US mainland.

But experts say such vessels are likely unfeasible for the North without external assistance in the near term.

North Korea has an estimated 70 to 90 diesel-powered submarines in one of the world’s largest submarine fleets, but they are mostly aging vessels capable of launching only torpedoes and mines.

South Korea’s military said the submarine unveiled by North Korea in September, the Hero Kim Kun Ok, did not look ready for operational duty and suggested the North was exaggerating its capabilities.

The submarine appeared to have at least 10 launch tubes possibly designed for missiles.

The South’s Joint Chiefs of Staff said the North would have needed to increase the size of the bridge and other parts of the original vessel to accommodate missile launch systems, but that the appearance of the vessel suggested it could “not be operated normally.”

Tensions on the Korean Peninsula have increased in recent months as Mr Kim accelerates his weapons development and issues provocative threats of nuclear conflict with the US and its Asian allies.