North Korea conducts test of underwater nuclear weapon system: KCNA

North Korea conducted a test of its underwater nuclear weapon system in response to the United States, Japan and South Korea's "joint military exercises," state-run media Korean Central News Agency said on Friday.

The "joint maritime exercises" conducted by the three countries, which took place in the waters around Jeju Island, have "become a cause of further destabilization of the regional situation" and a "serious threat" to the security of North Korea, according to an unnamed spokesperson at the North Korean Ministry of Defense, per KCNA.

As a countermeasure to the "serious threat," North Korea tested its "Haeil 5-23" system, which has been under development in the waters of the East Sea, according to the KCNA.

It's unclear how long the system has been under development.

Last summer, North Korea test-fired a series of intercontinental ballistic missiles that fell into the East Sea, and were deemed a "clear violation" of U.N. Security Council's resolutions.

The U.S., South Korean and Japanese militaries have been carrying out joint naval exercises, involving the USS Carl Vinson aircraft carrier this week, according to the Associated Press.

ABC News' Ellie Kaufman contributed to this report.