The U.S. military has detected "highly unusual" North Korean submarine activity, just days after the rogue nation conducted its second test of an intercontinental ballistic missile.

CNN said the submarine activity was evidence of an "ejection test," which examines a missile's "cold-launch" system.

An ejection test uses high-pressure steam to propel a missile out of the launch canister into the air before its engines ignite, CNN reported. This prevents damage to the submarine launching the missile.

This would be North Korea's third ejection test this month, and demonstrates that Pyongyang is making progress in developing its submarine launch skills.

The latest test comes after Friday's intercontinental missile launch by North Korea that traveled about 600 miles before falling in the Sea of Japan. Experts say the test showed a capability of reaching the U.S.