North Korea has been engaged in “highly unusual and unprecedented levels” of submarine activity in the wake of its second intercontinental ballistic missile launch this month, according to analysis by the U.S. military.

Speaking with CNN, an unnamed U.S. defense official said Monday the military also detected evidence of an “ejection test” Sunday at the Sinpo Naval Shipyard on the country’s eastern seaboard. Sinpo functions as a center of North Korea’s submarine-launched ballistic missile program.

Ejection tests like Sunday’s — of which CNN says there have been four this year, three in July alone — test the efficacy of a launch component critical to developing submarine missile launch capability. According to U.S. intelligence assessments, Pyongyang’s submarine fleet comprises about 70 subs, but few are thought to be new or sophisticated enough to fire missiles and the submarine-based launch program is still in its nascent stages.

Earlier in July CNN reported that one of the older class of North Korean subs, a Romeo, had sailed some 62 miles out to sea in international waters. The U.S. and South Korea slightly raised their alert level in response to its unusual deployment and range.

Read More: North Korea: There Is No Good Option

Pyongyang fired either a KN-11 or a Pukguksong-1 missile from a submarine last summer, in what officials believe was the country’s first successful submarine missile test.

North Korean leader Kim Jong Un claimed last week that “the entire U.S. territory is within our shooting range” after its latest missile launch, further ratcheting up tensions with Washington.

[CNN]

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Write to Joseph Hincks at joseph.hincks@time.com.