WASHINGTON -- Recent satellite photos have shown "somewhat unusual" activity at a North Korean submarine shipyard, including the repositioning of a missile test barge, though its purpose remains unclear, a US think tank said Tuesday.

Citing the recent satellite imagery, Beyond Parallel, a project of the Center for Strategic and International Studies, published an analysis on its recent observation of the shipyard in the eastern coastal city of Sinpo where an experimental ballistic missile submarine is docked.

Sometime between Jan. 5 and Jan. 8, an infiltration mother ship in the secure boat basin at the shipyard exchanged positions with the submersible missile test stand barge that has been berthed here since about 2014, the think tank said.

The infiltration mother ship is a vessel that the North previously used to deploy agents on high-speed landing craft for infiltrations into South Korea and Japan, and such ships have occasionally been observed at the shipyard for maintenance and repairs, according to the think tank.

It also noted developments related to the position of a telescoping crane at the shipyard.

"New and somewhat unusual activity has been observed around the 8.24 Yongung experimental ballistic missile submarine (SSBA) at the Sinpo South Shipyard's secure boat basin during the past four weeks," Beyond Parallel said.

"At this time, it is unclear whether the recent activity is an indicator of continuing work on the 8.24 Yongung SSBA, new work being undertaken on an infiltration mother ship, an ongoing deception program, or a combination of these and other reasons," it added.

The think tank noted that the submarine in question plays a "critical" role in the North's ongoing development of submarine-launched ballistic missiles and is an "indispensable" tool for hands-on training of new submariners. (Yonhap)