The military in the United States is maintaining a tight watch over what they call activity that is strange and unprecedented following reports of ejection tests launching ballistic missiles from submarines, which have been successful.
Analysts have stated that this is a vital step forward.
North Korea carried out three ejection tests at the Sinpo Naval Shipyard in July, while another test was undertaken earlier on in the year at the Maritime Research Institute of the Academy of National Defense Science in North Korea.
NORTH KOREA MAY BE ABLE TO ATTACK MOST OF CONTINENTAL US
The report follows on the latest North Korea testing of intercontinental ballistic missiles.
This testing showed Pyongyang might be able to attack the majority of the continental United States.
The recent ejection test was to test out a cold-launch system that would be needed for the propulsion of a missile from the launch tube of a submarine, before the ignition of the engine and so providing the submarine with protection.
The tests that were based on the ground coincided with a Romeo-class submarine belonging to North Korea, carrying out what had been described as activities that were unusual.
The tests were on the peninsula's east coast, and they continued over one week during late July.
THE UNITED STATES AND SOUTH KOREA BOTH MONITORING ACTIVITIES OF NORTH KOREA