Washington, Aug 1 (IANS): The US military has detected "highly unusual and unprecedented levels" of North Korean submarine activity and evidence of an "ejection test", a defence official told 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. It helps prevent flames and heat from the engine from damaging either the submarine, submersible barge or any nearby equipment used to launch the missile.

Carried out on land at Sinpo Naval Shipyard, Sunday's ejection test is the third time in July -- and fourth this year -- that North Korea has conducted a trial of the missile component that is critical to developing submarine launch capabilities, the official said late Monday.

This development comes after North Korea "successfully" test-fired a second intercontinental ballistic missile on July 28.

Pyongyang claimed the missile had a range to hit major US cities.