The U.S. military has reportedly detected what it describes as multiple “ejection tests” involving North Korean submarines.

That's according to CNN, which began citing Pentagon officials Monday as saying they've been detecting “highly unusual and unprecedented levels” of North Korean submarine activity.

The officials said the North conducted "cold-launch" ejection tests on Sunday at its Shinpo Shipyard on its east coast. 

So-called "cold-launch" ejection tests practice using pressurized steam to propel a missile into the air before its fuel ignites, so that the launch process doesn't damage the submarine.

The U.S. cable media outlet says the North has conducted four ejection tests this year, with three of them happening last month alone.

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