WASHINGTON -- New satellite imagery of Sinpo South Shipyard has shown "unusual movement" of North Korea's experimental ballistic missile submarine that may indicate a possible submarine-launched ballistic missile (SLBM) test in the future, a US monitor said Wednesday.

"Satellite image collected on March 22 shows unusual movement of the 8.24 Yongung (August 24th Hero) experimental ballistic missile submarine (SSBA) within the secure boat basin," Beyond Parallel, a project of the Washington-based Center for Strategic and International Studies, said in a report released on its website.

The US monitor said the submarine was seen moved out from the underneath a canopy that usually covers it.

"The exact purpose for moving the submarine is unknown, but it is likely related to ongoing modifications, continued repair work, preparations for an upcoming submarine-launched ballistic missile SLBM test, a component of a strategic deception plan, or a combination of these and other reasons," it said.

"The 8.24 Yongung plays a critical role in the under-way development of SLBMs, ballistic missile submarine technology, and operational procedures. It is also an indispensable tool for the hands-on training of new submariners," it added.

North Korea fired an intercontinental ballistic missile (ICBM) last week, marking its first ICBM launch in over four years since November 2017.

Pyongyang had maintained a self-imposed moratorium on nuclear and long-range missile testing since then, but said in January that it may restart all "temporarily suspended activities."

Earlier reports said the North may be repairing underground tunnels at the Punggye-ri nuclear test site that Pyongyang purportedly dismantled in 2018, possibly indicating an upcoming resumption of its nuclear tests.

To date, North Korea has conducted six nuclear tests, with the last one held in September 2017. (Yonhap)