In a revelation that has strategic implications for Japan, analysis of satellite imagery shows the existence of North Korea's second submersible test-stand barge — a sign that the nuclear-armed country could be ramping up development of its submarine-launched ballistic missile (SLBM) program.

According to the analysis released Monday by the 38 North website, a project of the U.S.-Korea Institute at Johns Hopkins University, the barge was identified in commercial satellite images taken April 19 of the Nampo Naval Shipyard on the country's west coast.

The isolated nation already operates one barge on the country's eastern coast, at the Sinpo South Shipyard, from where it has conducted at least four — but as many as six — test-launches of the Pukguksong-1, or KN-11, SLBM since 2014, when that barge was first seen.