North Korea said Friday it conducted an "important test of its underwater nuclear weapon system" in the Sea of Japan in response to joint military exercises by the United States, South Korea and Japan.

The test was of the "Haeil-5-23" system, which is still under development, North Korea's Defense Ministry said in a statement, according to the official Korean Central News Agency. The statement did not give further details of the test.

"Our army's underwater nuke-based countering posture is being further rounded off and its various maritime and underwater responsive actions will continue to deter the hostile military maneuvers of the navies of the U.S. and its allies," the KCNA report quoted the statement as saying.

File photo shows an unmanned nuclear underwater attack craft being tested by North Korea in March 2023. (KCNA/Kyodo)

Last year, Pyongyang claimed it had successfully tested an underwater nuclear attack drone designed to generate tsunami to destroy enemy ships and ports.

The statement said the three-day trilateral drills conducted in waters off South Korea's Jeju Island from Monday are "an act of seriously threatening the security" of North Korea. "We will never tolerate the reckless military confrontation hysteria," the ministry said, according to the report.

Meanwhile, North Korean Ambassador to China Ri Ryong Nam met Friday with Liu Jianchao, head of the Chinese Communist Party's International Liaison Department, and agreed that the two countries will work to deepen their relations as this year marks the 75th anniversary of the establishment of bilateral diplomatic ties.

Ri expressed Pyongyang's firm support to China's struggle to safeguard its core interests related to Taiwan and Hong Kong, among others, the Communist Party said.

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