North Korean leader Kim Jong-un has called the North's launch of a ballistic missile from a submarine "the greatest success," saying his country has full capability to carry out nuclear attacks, Pyongyang's state media said Thursday.

The North's leader observed the launch of the submarine-launched ballistic missile Wednesday, saying it was "the greatest success and victory," according to the official Korean Central News Agency.

Kim said the results of the test-fire showed North Korea "joined the front rank of the military powers fully equipped with nuclear attack capability," the KCNA reported.

The South Korean military said the missile flew about 500 kilometers toward Japan, making the longest flight by such a missile launched by the North.

A military source said the missile, launched at a higher angle, could have flown more than 1,000 km if it was fired off at a regular angle, and is seen as technical advances in the North's missile program.

The KCNA said the test-fire was conducted under a high-angle fire system using a solid fuel engine, claiming that it proved the country "perfectly" possesses the core technology of the SLBM.

On Monday, Seoul and Washington kicked off their annual joint military drill, which Pyongyang has long denounced as a rehearsal for northern invasion.

Tension is running high on the divided peninsula as North Korea threatened on Monday to wage a "pre-emptive nuclear strike" on South Korea and the US against their military exercise.

The KCNA said the North's leader stressed the need to make efforts to mount nuclear warheads on ballistic missiles and develop means of their delivery in order to cope with nuclear war with the US

North Korea has insisted that its nuclear weapons development is an act of nuclear deterrence for self-defense in the face of what it claims is Washington's hostile policy toward Pyongyang.

"North Korea has yet to fully carry out orders by the North's leader (over nuclear and missile programs). In that sense, the country appears to focus on developing means of delivery," said an official at Seoul's unification ministry.

In March, Kim said that a nuclear warhead test and a test-fire of ballistic missiles able to carry nuclear warheads will be conducted in a short time.

Wednesday's launch marked the sixth time that North Korea has tested its SLBM capabilities after its first attempt in May 2015.

It also marked the third SLBM test launch this year.

North Korea's purported success of the SLBM launch could serve as a fresh threat to regional security, as it is difficult to detect ballistic missiles when launched underwater.

North Korea is seeking to develop a nuclear-tipped intercontinental ballistic missile (ICBM) capable of hitting targets on the US mainland.

If an SLBM is capable of carrying a small nuclear warhead, it can pose a formidable threat, as the North would be able to hit targets theoretically from anywhere and in a blitzkrieg manner, experts said.

Seoul and Washington are skeptical about the North's claim over the success of developing miniaturized nuclear bombs. But experts said that Pyongyang seems to have made significant progress in making a nuclear warhead small enough to put on a missile.

Since the North's leader took office in late 2011, North Korea has test-fired more than 30 ballistic missiles, including an intermediate-range Musudan missile, which theoretically can fly as far as the US territory of Guam.

The South Korean military had expected North Korea to be able to deploy SLBMs for combat use within two or three years, but some experts said that the latest launch could enable the North to move forward its deployment to as early as late this year.

A 2,000 ton-submarine being used by North Korea is believed to be able to operate underwater for just a few hours and to possess only one launcher for a ballistic missile.

Analysts said that it would take some time for North Korea to improve its existing submarine or build a large, nuclear-powered submarine for combat use.

"North Korea seems bent on testing SLBMs as it would be not easy for the country to develop ICBMs," said a military official.

North Korea is banned from using ballistic missile technology under relevant United Nations Security Council resolutions.

The UNSC slapped its toughest sanctions on the North over its fourth nuclear test and long-range rocket launch early this year. (Yonhap)