WASHINGTON - Satellite images taken this month of a North Korean naval shipyard indicate Pyongyang is pursuing an "aggressive schedule" to build its first operational ballistic missile submarine, a US institute reported on Thursday.Washington-based 38 North, a North Korea monitoring project, cited images taken on November 5 showing activity at North Korea's Sinpo South Shipyard."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," 38 North said in a report.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 center 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 United States, 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. It said the submarine was likely a larger successor to North Korea's single experimental ballistic missile submarine.North Korea has conducted dozens of missile tests this year and its largest and biggest nuclear test on September 3. The past two months have seen a relative lull and it has not tested a missile since firing one over Japan on September 15.Another article in The Diplomat last month quoted a US government source as saying that North Korea had tested a new solid fuel engine sometime between October 15 and October 21. US intelligence officials have declined to comment on this.