A U.S.-based North Korea monitoring website has assessed that North Korea is continuing construction of what may be a Sinpo-class ballistic missile submarine. 

38 North published the assessment on Wednesday, citing recent commercial satellite imagery of the east coast Sinpo South Shipyard. The North is known to have one other Sinpo-class ballistic missile submarine in service.

The analysis says movement patterns of parts and equipment in the staging area near construction facilities indicate continuing construction of what may be a Sinpo-class ballistic missile submarine. 

The website also said that minor improvements were being made to a secure boat basin at the shipyard, including the installation of 12 probable davit arm cranes spaced at regular intervals along the waterside sometime between April 11 and May 5. 

It speculated these cranes would facilitate loading lightweight supplies and equipment down the hatches of a submarine and onto a submersible launch barge.

In April 2016, North Korea launched a ballistic missile from a submarine ten meters below sea level, but it exploded mid-air after flying around 30 kilometers.

38 North and other outlets have speculated since then that the North continues to develop its submarine-based weapons technology.