A North Korean shipyard capable of building ballistic missile submarines appears to be somewhat active, but it's unclear whether a new submarine is being built, according to a US website monitoring the regime.

38 North, a project of the Stimson Center think tank, said it has observed in recent commercial satellite imagery "a low level of activity" at the Sinpo South Shipyard, the most probable place North Korea would build its first operational ballistic missile submarine.

"Movement of parts and equipment consistent with a shipbuilding campaign first observed last year appear to be ongoing," the website said in an article posted Thursday. "However, there is no conclusive evidence that a new submarine is being built or if so, what type of ship is under construction."

Activity is visible in the parts storage yard, where a gantry crane that was being erected in imagery from May is now operational, according to the article. Parts and components also continue to be moved into and out of the yard.

Taken as a whole, these observations suggest an ongoing shipbuilding program or preparations for a new one," it said.

In the secure boat basin, there are indications dredging is under way, which could be for simple maintenance purposes or to create more clearance for the SINPO-class experimental ballistic missile submarine. It could also be to create deeper anchorage for a larger submarine or vessel.

Construction appears to be continuing at a slow pace for a new construction or maintenance hall and an L-shaped pier. But the higher than normal levels of activity that were observed in the shipyard's northern support area from December to May seemed to have stopped by July.

38 North was the first to report the dismantling of a North Korean missile engine testing site, citing satellite imagery from July 20.

The dismantlement was a verbal pledge made by North Korean leader Kim Jong-un to US President Donald Trump at their June summit in Singapore. (Yonhap)