An online outlet on North Korea said it has detected signs that Pyongyang has likely begun construction of a new submarine that is suspected of being the same class as the North’s Hero Kim Kun Ok ballistic missile submarine(SSBN) unveiled last September.

Citing recent commercial satellite images from the Sinpho South Shipyard, the U.S. monitoring website 38 North said on Wednesday that since late last year, hull components and other pieces of equipment were brought into the shipyard.

The first jig, which is a piece of submarine construction equipment, was observed through imagery from late December, with two more appearing by mid-February. Images from March 31 showed three hull sections, each measuring five-point-six, six-point-four and seven meters in diameter.

38 North said the hull section measurements suggest that the vessel under construction is likely the same class as the Sinpo-C class Hero Kim Kun Ok SSBN, which Pyongyang claims is its first "tactical nuclear attack submarine."

Imagery from April 27 also showed that a panel section above the Hero Kim Kun Ok's sail had been removed, while a mobile crane was seen parked with its boom in the stored position.