By Jack Kim

SEOUL (Reuters) - North Korean leader Kim Jong Un supervised the test-firing of a submarine-launched ballistic missile and declared it "the greatest success," which puts the country in the "front rank" of nuclear military powers, official media reported on Thursday.

North Korea fired a submarine-launched ballistic missile (SLBM) on Wednesday which flew about 500 km (300 miles) towards Japan. The South Korean government and experts said the launch showed technical progress in the North's SLBM program.

"A test-fire of strategic submarine-launched ballistic missile was successfully conducted under the guidance of supreme commander of the Korean People's Army Kim Jong Un," the North's official KCNA news agency said.

"He appreciated the test-fire as the greatest success and victory," KCNA said.

"He noted with pride that the results of the test-fire proved in actuality that the DPRK joined the front rank of the military powers fully equipped with nuclear attack capability."

DPRK, the Democratic People's Republic of Korea, is North Korea's formal name.

North Korea has conducted a spate of military technology tests this year, including a fourth nuclear test in January and numerous ballistic missile launches, in defiance of U.N. Security Council sanctions that were tightened in March.

North Korea said this year it had miniaturized a nuclear warhead to fit on a ballistic missile but outside experts have said there is yet no firm evidence to back up that claim or show it had mastered the technology to bring a live warhead back into the atmosphere and guide it to strike a target.

North Korean state television on Thursday showed video clips of the launch of a missile from underwater at dawn, and still photographs of Kim on the dock at a port as a large crane unloaded an object onto a submarine.

Kim is also seen jubilantly celebrating with military aides in photographs carried by the official Rodong Sinmun newspaper.

REACHED JAPAN DEFENCE ZONE

The Washington-based 38 North project said in a report that the missile was launched from the North's sole experimental missile submarine and a satellite photograph taken on Monday showed final preparations, likely after the missile had already been loaded onto the submarine using a heavy construction crane.

The test showed the solid-fuel missile's control and guidance system as well as the atmospheric re-entry of the warhead all met operational requirements, KCNA said.

The South Korean and U.S. militaries said the missile was fired from near the coastal city of Sinpo, where a submarine base is located. Japan said the missile reached its air defense identification zone, the first time by a North Korean missile.

The UN Security Council met behind closed doors on Wednesday at the request of the United States and Japan to discuss the launch. Deputy Russian U.N. Ambassador Petr Iliichev said the United States would circulate a draft press statement.

The meeting comes after the Security Council was unable to condemn a missile launch by the North earlier this month that landed near Japan because China wanted the statement to also oppose the planned deployment of a U.S. missile defense system in South Korea.

China said on Wednesday that it opposes the North's nuclear and missile programs. It had been angered by what it views as provocative moves by the United States and South Korea on the decision to deploy the THAAD (Terminal High Altitude Area Defence) anti-missile system in South Korea.

(Additional reporting by Minwoo Park in Seoul and Michelle Nichols at the United Nations; Editing by Raju Gopalakrishnan)