A image acquired on Tuesday, showing the No. 841 Hero Kim Kun Ok SSB from the portside. (Center for Strategic&International Studies)

A US monitor on Thursday reported recent North Korean ballistic missile submarine movements at a shipyard on the country's east coast, saying they may presage the start of submarine trials this summer or preparations for a submarine-launched ballistic missile test.

Citing recent satellite imagery, Beyond Parallel, a project of the Center for Strategic and International Studies, released its monthslong analysis of the movements of the Hero Kim Kun Ok submarine, an experimental submarine, and the submersible missile test stand barge at a shipyard in Sinpo.

Launched in September, Hero Kim Kun Ok is known as the country's first true ballistic missile submarine.

"This overall level of activity may portend the commencement later this summer of sea trials for the Hero Kim Kun Ok, preparations for the resumption of SLBM/SLCM flight testing, or both," the monitor said in the analysis. SLCM stands for a submarine-launched cruise missile.

"Going forward, the level of activity and success these vessels achieve will heavily influence future North Korean development of SSBs, SLBMs, SLCMs, and submarine technology, designs, and operational procedures," it added. SSB stands for a ballistic missile submarine.

Key movements at the shipyard involved Hero Kim Kun Ok being moved from under the security canopy in the secure boat basin to the graving dock.

"The most likely explanation for the Hero Kim Kun Ok's presence in the graving dock is that it is a component of the submarine's final construction and fitting out," it said.

"The latter would include the final installation and testing of systems such as electrical, pumping, and communications, suggesting forthcoming sea trials and being declared operational." (Yonhap)