North Korea has continued to build a new ballistic missile submarine at its Sinpo shipyard, a US monitor said Thursday, citing recent commercial satellite imagery.

38 North, which monitors activities inside the North Korean regime, said satellite imagery of the shipyard from Aug. 26 and Sept. 23 showed the movement of parts and equipment near the construction hall, indicating construction has continued since North Korean leader Kim Jong-un's reported visit to the site in late July.

However, the type of submarine under construction and whether it is nearing completion remain unclear, the monitor added on its website.

Photos of Kim's visit to the shipyard suggested the submarine he inspected was a modified Soviet ROMEO-class conventional submarine, not the SINPO-C class ballistic missile submarine widely believed to be under construction, it said.

Preparations for a preliminary test could also be seen.

"The presence of a cylindrical canister and support vehicles near the submersible test barge berthed at the secure boat basin strongly indicates that preparations are underway for the testing of a dummy round from its missile launch ejection system," the monitor said. "This test is routinely done prior to actual missile launches, to ensure that the missile can be ejected at the proper speed and angle before committing to ejection and launch from the submarine."

Meanwhile, analysis of the imagery produced measurements consistent with a missile launch canister for the Pukguksong-1 (KN-11) submarine-launched ballistic missile.

The Pukguksong-1 missile is believed to be 8.6 meters long and 1.5 meters in diameter, according to 38 North. (Yonhap)