North Korea appears to be trying to equip a submarine to make it capable of firing missiles, a U.S think tank said Thursday, warning such hard-to-detect, missile-capable submarines would pose significant threats to South Korea.

The website 38 North, run by the U.S.-Korea Institute at the Johns Hopkins School of Advanced International Studies, said it reached the assessment based on commercial satellite imagery taken of the Sinpo South Shipyard on the east coast between July and December.

Imagery from Dec. 18 shows a rectangular opening, about 4.25 meters long and 2.25 meters wide, on top of the conning tower of a submarine, and the opening is believed to be designed to house one to two small vertical missile launch tubes, the website said, citing analysis by Joseph Bermudez, an expert on satellite imagery.

The imagery also showed workers moving around the area, equipment stored on the deck and a heavy-lift construction crane, the website said, adding that the only reasonable explanation for the crane's presence is "continuing to work on fitting out the submarine."

"North Korea's development of a submarine-launched missile capability would eventually expand Pyongyang's threat to South Korea, Japan and US bases in East Asia, also complicating regional missile defense planning, deployment and operations," the website said. "Submarines carrying land-attack missiles would be challenging to locate and track, would be mobile assets able to attack from any direction, and could operate at significant distances from the Korean peninsula."

It is unclear what missile system would be used in a ballistic missile submarine, but possibilities include a shorter naval version of the Musudan intermediate-range ballistic missile, a Rodong medium-range ballistic missile and naval versions of the solid-fueled KN-02 short-range ballistic missile, it said.

"Nevertheless, such a threat is not present today. Moreover, an effort by Pyongyang to develop an operational missile-carrying submarine would be an expensive and time-consuming endeavor with no guarantee of success," the website said.

North Korea is believed to have developed advanced missile technologies through a series of test launches. In its latest rocket launch, conducted in late 2012, the North succeeded in putting a satellite into orbit aboard a long-range rocket.

The test sparked fears that the North has moved closer to ultimately developing nuclear-tipped missiles that could potentially reach the mainland U.S. The country has so far conducted three underground nuclear tests in 2006, 2009 and 2013. (Yonhap)