The US-Korea Institute at the Johns Hopkins School of Advanced International Studies yesterday said that December 23 commercial satellite imagery of a naval shipyard at the east coast site of Sinpo suggests that the submarine used in the test remains seaworthy and that there may be new testing activity.

The imagery also shows North Korea is constructing facilities that could accommodate the building of bigger submarines, according to the analysis published by 38 North, the institute's website.

Missiles launched from submerged vessels would be harder to detect that land-based ones, but the institute says North Korea likely remains years away from having an operational system.

International concern has deepened over North Korea's development of nuclear weapons and missiles to deliver them.

Last May, North Korea said that it successfully test-fired a ballistic missile from a submarine in what it touted as a display of the country's advancing military capabilities, although some experts questions the authenticity of the photos Pyongyang publicised of the test.

"North Korea is clearly serious about developing a sea-launched ballistic missile that would pose a new threat to countries in the region," said Joel Wit, a former State Department official and editor of 38 North.

Latest News from World News Desk