North Korea is developing a submarine capable of launching ballistic missiles, a U.S.-based web magazine claimed Tuesday, escalating concerns here over missile and nuclear threats from the communist state.

Citing two U.S. defense officials, Washington Free Beacon said in an article that a missile launch tube on a submarine was recently observed by U.S. intelligence agencies ― an indication that the North was building a ballistic missile submarine.

Expressing doubts over the report, a senior South Korean Navy official told The Korea Herald that Pyongyang’s military technology has yet to reach the level of being able to deploy submarine-launched ballistic missiles, or SLBMs.

The report said that the North’s new missile submarine could be a modification of one of North Korea’s Russian- or Chinese-design Romeo-class diesel submarines or of a Soviet-era Golf-class missile-firing submarine purchased by Pyongyang for scrap metal in the mid-1990s.

The influential Jane’s Fighting Ships, an annual reference book on the world’s warships, revealed in May 1994 that the North had purchased 40 decommissioned submarines from Russia, including several Golf-class and Romeo-class submarines.

The webzine also noted that the North covertly obtained several Soviet-made SS-N-6 SLBMs. The missiles have been adapted into the reclusive state’s new intermediate-range missiles, the report added.

The SS-N-6 missile measures 9.65 meters in length and 1.5 meters in diameter. Its launching weight is around 14 tons. Having said this, Seoul officials remained skeptical about the North possessing submarines capable of firing large-scale missiles.

“If you want to mount a ballistic missile, the submarine should weigh between 8,000 tons and 10,000 tons. Even the Golf-class submarine weighs between 2,700 tons and 3,500 tons,” said a South Korean Defense Ministry official.

A South Korean Navy official also argued that given the North’s current submarine capabilities, it is highly unlikely that the North had deployed SLBMs.

“With the current tonnage of the North Korean submarines, they can only carry torpedoes,” the official told The Korea Herald.

“On top of that, what should be done before placing ballistic missiles on submarines is to ensure that they have adequate information on the targets. But they don’t have a satellite or any system to determine the direction of the SLBMs.”

Should it be confirmed that the North has deployed SLBMs, it would constitute a serious nuclear threat. Nuclear experts consider strategic bombers, intercontinental ballistic missiles and SLBMs to be the components of a “nuclear triad” needed to ensure a nation’s nuclear deterrence.

The communist state’s submarines outnumber its southern neighbor’s, though they are old and equipped with outdated weapons. The North is believed to have some 70 submarines including some 20 1,800-ton Romeo-class submarines and about 40 325-ton Sangeo-class submarines.

South Korea has been seeking to bolster its antisubmarine capabilities since its corvette Cheonan was torpedoed by a North Korean midget submarine in the West Sea in March 2010. The attack killed 46 sailors, but Pyongyang denies responsibility.

By Song Sang-ho (sshluck@heraldcorp.com)