North Korea is proceeding with its development of a submarine-launched ballistic missile despite reports of a failed test several weeks ago, a U.S. research institute said Tuesday, according to AP. The U.S.-Korea Institute at the Johns Hopkins School of Advanced International Studies said that Dec. 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 publicized of the test. South Korea's Defense Ministry said the missile traveled about 150 yards. (135 meters). In late November, South Korean media reported that another test had failed and that the North's experimental missile-launching submarine may have been damaged.