Story highlights An ejection test examines a missile's "cold-launch system," which uses high pressure steam to propel a missile out of the launch canister

Carried out on land Sunday at Sinpo Naval Shipyard, this is the third such test this month

Washington (CNN) The US military has detected "highly unusual and unprecedented levels" of North Korean submarine activity and evidence of an "ejection test" in the days following Pyongyang's second intercontinental ballistic missile launch this month, a defense official told CNN on Monday.

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. That 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 this month -- and fourth this year -- that North Korea has conducted a trial of the missile component that is critical to developing submarine launch capabilities, according to the US defense official.

US officials also noted that a North Korean Sang-O submarine was operating in the Yellow Sea and the length of its deployment was notable. Two Romeo submarines were detected in the waters off Japan -- each one operating in the area for about a week.

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