Washington-based 38 North, a North Korea monitoring project, published images taken on November 5 showing activity at North Korea's Sinpo South Shipyard.
Its report stated: "The presence of what appear to be sections of a submarine's pressure hull in the yards suggests construction of a new submarine, possibly the SINPO-C ballistic missile submarine - the follow-on to the current SINPO-class experimental ballistic missile submarine."
The report said that throughout 2017 there had been continued movement of parts and components into and out of two parts yards adjacent to the constructions halls in the centre of the shipyard.
It said the November 5 images showed two large circular objects that could be sections of a submarine's pressure hull. It said these appeared larger than those for North Korea's ROMEO-class attack submarine.
Images of a test stand indicated continued testing of a mechanism for ejection launch of missiles from a submarine.
However, the report said no activity could be seen suggesting preparations for a new test of a submarine-launched missile.
North Korea has been working to develop a nuclear-tipped missile capable of reaching the US, sparking a major international crisis in which US President Donald Trump has said that all options are under consideration, including military ones.
North Korea is also thought to be working on a solid-fuel missile for submarine launches.
Last month, The Diplomat magazine quoted a US government source as saying US military intelligence had detected a new diesel-electric submarine under construction at Sinpo and dubbed it the Sinpo-C. READ MORE
