reports North Korea is building what is estimated to be a 3,000-ton submarine.
It is suggested it could be capable of launching ballistic missiles which can easily reach the United States and beyond.
A senior military spokesman said: "A large quantity of components presumably needed to build a 3,000-ton sub, which the North has been developing, have turned up recently at a shipyard in Sinpo, South Hamgyong Province.
"We've also spotted a floating dock deployed off Sinpo, apparently for a missile test."
'SUB PARTS SPOTTED FROM AIR' Satellite imagery taken in March shows cylindrical components.
It is speculated they are bulkheads which will be welded together to form the submarine's hull.
None of the components could be seen from space last year.
A South Korean military spokesman said: "This suggests that the North has kept building a new sub and testing a submarine-launched ballistic missile.”
This means construction has been underway just weeks after the June Singapore summit between Kim and US President Donald Trump and the one in Vietnam last February.
This suggests that the North has kept building a new sub and testing a submarine-launched ballistic missile.
South Korean military spokesman The North already has a 2,000-ton submarine. This is said to be capable of carrying only one ballistic missile.
Yet the new sub could deploy three or four.
This is still far behind the capability of other nuclear powers.
The Royal Navy for example has four 15,000 ton subs that each can launch 16 Trident nuke missiles.
The United States Navy, meanwhile, has 14 Ohio class boats, which are about similar size and firepower as the British ones.
But while the new sub would not match those possessed by existing nuclear powers, the building of the North Korean vessel would drastically increase its strategic strike capability.
Any launch would be hard to detect and it might be unclear which nation fired the missile.
Gen Terrence O'Shaughnessy, the commander of the US Northern Command, said in a hearing at a US Senate Armed Services Subcommittee the North's deployment of intercontinental ballistic missiles is "imminent".
Gen O'Shaughnessy said it is almost certain they are designed for an attack on the US mainland.
He said “He almost certainly has plans to use them against our homeland should a conflict erupt on the peninsula.”
North Korean leader Kim Jong-un appears to be forging ahead with his weapons programme Credit: Reuters
New satellite images of North Korea suggest it is rebuilding Sohae rocket launch site it had vowed to dismantle We pay for your stories! Do you have a story for The Sun Online politics team? Email us at tips @the-sun.co.uk or call 0207 782 4368. We pay for videos too. Click here to upload yours