WASHINGTON: Just days after North Korea conducted an Intercontinental Ballistic Missile (ICBM) test, the US military said it has detected "highly unusual and unprecedented levels" of submarine activity, CNN reported on Tuesday (Aug 1) citing a defence official.
There is also evidence that Pyongyang has conducted an "ejection test", the official said.
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", according to CNN.
The ejection test was the fourth this year and was conducted on land at Sinpo Naval Shipyard, the defence official added.
US President Donald Trump has been weighing how to respond to North Korea's latest missile test - a sore point between Washington and Beijing.
Trump has been critical of China, North Korea's closest ally, saying it should do more to rein in Pyongyang.
Experts have warned North Korea could field a nuclear-capable intercontinental ballistic missile able to strike America by next year, but Pyongyang must first overcome important technological hurdles.