The United States military said it has detected "highly unusual and unprecedented levels" of submarine activity, CNN reported on Tuesday citing a defence official. This comes just a few days after North Korea conducted an Intercontinental Ballistic Missile (ICBM) test.
The official added that there is also evidence that Pyongyang has conducted an "ejection test". According to CNN, an ejection test examines a missile's cold-launch system, "which uses high pressure steam to propel a missile out of the launch canister into the air before its engines ignite, preventing damage to the submarine or submersible barge that would launch the missile".
The defence official further added that the ejection test was the fourth this year and was conducted on land at Sinpo Naval Shipyard.
On Saturday, North Korea said that it conducted another successful test of an intercontinental ballistic missile (ICBM) on Friday. Reportedly, the missile proved its ability to strike America's mainland, drawing a sharp warning from US President Donald Trump.
US President Donald Trump has been weighing how to respond to North Korea's latest missile test, which is a sore point between Washington and Beijing.
China, the North's main ally, said it opposed North Korea's "launch activities that run counter to Security Council resolutions and the common wishes of the international community." However, Trump has been extremely critical of China and said that it should do more to rein in Pyongyang.
The experts have warned that North Korea could field a nuclear-capable intercontinental ballistic missile able to strike America by next year. But, in order to do that it has to first overcome important technological hurdles.