Satellite images of shipyards in the northeastern port city of Sinpo offer evidence of the construction of the new vessel, rsays esearchers from the Middlebury Institute of International Studies. The images show the construction of a large structure jutting out into the water from the quay.
Satellite photos taken earlier this month were compared with the images taken this week and show the structure has grown to about 100 metres in that time.
The researchers said North Korea’s conventional submarines are usually built-in structures much smaller than the one that appeared in satellite images.
A senior researcher from the Middlebury Institute said the structure could be used for the construction and maintenance of North Korea’s submarine-launched ballistic missile programme.
The claims support reports by North Korea’s state media which said leader Kim Jong-Un had inspected a newly-built submarine at an undisclosed location.
A Sinpo-class submarine is already thought to be in operation in North Korea, which launched what is believed to be a KN-11 ballistic missile into the Sea of Japan in 2016.
The Sinpo-class sub, however, only has one missile launch tube, leading experts to speculate that the country may well be attempting to build a sub with multiple launch tubes to increase devastation.
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If a nuclear-capable submarine is built, it would be more difficult for the US and its allies to track, according to the analysts.
A report from the Centre for Strategic and International Studies (CSIS)said: “The construction and commissioning of a true ballistic missile submarine capability would represent a significant advancement of the North Korean ballistic missile and nuclear threat and complicate defence planning in the region, given the difficulties of tracking and/or pre-emptively targeting such capabilities.”
North Korea has been stepping up its military activities in recent weeks and last week launched two short-range missiles just hours after Kim offered to resume nuclear talks with the US. The launches were the eighth since late July and the first since August 24.
The previous seven launches were short-range missile and rocket artillery systems that experts say would potentially expand its capabilities to strike targets throughout South Korea, including US military bases there.
Analysts said the latest launches could be designed to strengthen Kim’s negotiating hand ahead of fresh negotiations.
Dialogue between the two sides broke down last February when a summit between Donald Trump and his North Korean countepart ended early without resolution.
North Korea is widely believed to want the US to provide it with security guarantees and extensive relief from US-led sanctions in return for limited denuclearisation steps.
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While North Korea conducts missile tests for a range of purposes, including technical development and reassurance for the country’s defence establishment, the latest activity appears timed to send a message to Washington.
Daniel DePetris, a fellow at Washington-based think tank Defence Priorities, said the launches were probably the latest case of Kim turning to missile tests as diplomatic signalling.
He said: “Far be it from me to get inside Kim’s head, but the simplest answer may be the most accurate: North Korea is demonstrating what will happen if the US doesn’t come to the table with realistic proposals.”