North Korea's state media Korean Central News Agency reported on Friday it had conducted a test of its underwater nuclear weapon system in the sea off the Korean Peninsula's east coast.

The test of "Haeil 5-23," a system of underwater drones capable of covert nuclear attacks on naval forces and ports, was carried out in a show of protest against a joint maritime military drill of South Korea, Japan and the United States held earlier this week, the KCNA reported. The trilateral maritime drill involved nine warships, including the US Navy's nuclear-powered carrier, the USS Carl Vinson.

"The Underwater Weapon System Institute under the DPRK Academy of Defense Science conducted an important test of its underwater nuclear weapon system ‘Haeil-5-23’ under development in the East Sea of Korea," a North Korean Ministry of National Defense spokesperson was quoted as saying by the KCNA, in English. DPRK stands for the Democratic People's Republic of Korea, the official name of North Korea. The KCNA did not further specify where the test of underwater drones took place.

"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 US and its allies," KCNA also reported.

Haeil conducted its first test in March 2023, according to the North's state media, and such test took place four times, including Friday's one.

South Korea's Defense Ministry said in a statement Friday afternoon that any direct provocation of North Korea will lead to "(South Korea's) overwhelming response by retaliating immediately and strongly until the end," adding its armed forces are in close coordination with US Forces to stand united against any military provocations from the North.