SEOUL (Reuters) - North Korea said on Saturday it had successfully test-fired a ballistic missile from a submarine, a step that would mark significant progress in the secretive state’s military capabilities.

North Korean leader Kim Jong Un watches the test-fire of a strategic submarine underwater ballistic missile (not pictured), in this undated photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang on May 9, 2015. REUTERS/KCNA

It could pose a new threat to South Korea, Japan and the United States, which have tried to contain North Korea’s growing nuclear and missile strength, military experts said.

The North’s leader, Kim Jong Un, oversaw the test-launch from an offshore location as the submarine dived and “a ballistic missile surfaced from the sea and soared into the air, leaving a fiery trail of blaze,” the official KCNA news agency said.

“Through the test, it was verified and confirmed that the underwater ballistic missile launch from a strategic submarine fully achieved the latest military, scientific and technical requirements.”

North Korea is under United Nations sanctions banning it from developing or using ballistic missile technology.

The United States would not comment on the reported test but said launches using ballistic missile technology were a “clear violation” of U.N. Security Council resolutions.

“We call on North Korea to refrain from actions that further raise tensions in the region and focus instead on taking concrete steps toward fulfilling its international commitments and obligations,” a State Department official said in an email.

The KCNA report did not mention the date or the exact location of the test, but a separate KCNA dispatch on Saturday said Kim gave field guidance at a fishery complex in Sinpo, a port city on the country’s east coast and the location of a known submarine base.

ADVANCED CAPABILITIES

The test, if verified, would mean North Korea can move its missiles within range of the United States, said Korea analyst Victor Cha of the Center for Strategic and International Studies in Washington.

“The test shows that North Korea missile capabilities are advancing without any constraints now despite a bevy of nonproliferation sanctions applied by the United Nations,” he said in an email.

A South Korean military expert who saw still photos of the launch in the North’s media said they appeared to show a ballistic missile being fired from a submarine in a “cold launch” through an ejection mechanism, a key element in a submarine launch system.

“The potential of this is that existing missile defense against the North can be rendered useless,” said Shin In-kyun, who runs the Korea Defense Network, an independent forum.

Such missile defense systems are positioned to look at the North, not at “submarines that could be south of Jeju or near Guam”. Jeju is a South Korean island.

Shin said a full deployment of the submarine-based missile system would still require a functional guided propulsion mechanism that can carry the missile vehicle from the water surface to a target.

The North has also yet to demonstrate it has successfully miniaturized a nuclear warhead to be fitted on a delivery vehicle for deployment, according to experts.

South Korea did not have an immediate comment on the report on the submarine launch.

MILITARY BOASTS

In January, Johns Hopkins University’s U.S.-Korea Institute said on its website, 38 North, that satellite imagery showed possible evidence of work on vertical launch tubes on a submarine that could be for ballistic missiles.

The vessel could serve “as an experimental test bed for land-attack submarines”, it said, although it cautioned such a test would be expensive and time-consuming “with no guarantee of success”.

North Korea’s state media often boasts of successful military and space accomplishments, including the launch of a functional communications satellite, which are not independently verified by outside experts.

It is believed to have launched a long-range rocket and put an object into orbit in December 2012, defying scepticism and international warnings not to pursue such a program, which could be used to develop intercontinental missiles.

South Korea’s military said later on Saturday the North had fired three land-based cruise missiles from a separate location on its east coast into the sea with a range of about 120 km (70 miles).

The North has an arsenal of land-based ballistic missiles and last test-fired a mid-range missile in March last year, drawing further condemnation from the international community.