The US military is monitoring "highly unusual and unprecedented levels" of activity by North Korea's submarine force, with reports of a series of successful "ejection tests" of submarine-launched ballistic missiles described by analysts as a "critical step forward".

Three ejection tests were carried out on land at Sinpo Naval Shipyard in July alone, US officials told CNN. A further test was carried out at the base, which is the headquarters of the Maritime Research Institute of North Korea's Academy of National Defence Science, earlier in the year.