External construction of a submarine assembly hall at North Korea's main submarine port is now complete in a project that would enable Pyongyang to build much larger ballistic missile submarines, a U.S. monitoring website said Wednesday.

The website 38 North, dedicated to monitoring and analyzing the communist nation, released the assessment based on satellite imagery of the North's Shinpo Shipyard, the homeport of North Korea's Gorae-class ballistic missile submarine program.

The imagery was taken after the North's latest submarine-launched ballistic missile test on July 9.

"Work on the launching way (i.e., ramp) and construction hall, 360 meters south of the secure boat basin, is now externally complete," 38 North said in a report. "The status of work inside the hall remains unclear, but when it is finished North Korea will be able to build and launch new submarines much larger than the existing GORAE-class including a new class of ballistic missile submarines."

The imagery also shows that the Gorae-class submarine is berthed at its normal location in the secure boat basin and probable post-test maintenance activity is being undertaken on the submarine with a heavy-lift crane and what appears to be a small truck or shipping container present dockside, according to the report.

Also in the secure boat basin are two mother ships, approximately 30 meters long, specialized craft used to transport intelligence agents and special operations troops on infiltration missions against South Korea and Japan, 38 North said.

The North's latest SLBM test came a day after South Korea and the U.S. jointly announced a decision to deploy the THAAD missile defense system in the South to better defend against growing nuclear and missile threats from the North.

The North carried out its first SLBM test in May last year and conducted at least two more tests during the year. The North's SLBM capability, if fully developed, would pose a serious threat because its mobile nature would make it very difficult to detect preparations for a launch. (Yonhap)