In response to military exercises conducted by American, South Korean, and Japanese forces, North Korea is publicizing an “underwater nuclear weapon system” that it claims should “strike horror into their hearts.” 

The North Korean boast, put out in English by Pyongyang’s Korean Central News Agency, was the latest blast in a rhetorical blitzkrieg in which the North’s leader, Kim Jong-un, has attacked South Korea as the North’s foremost enemy, has ruled out any chance of unifying the peninsula  and shut down agencies that had anything to do with North-South relations.

The test of Haeil-5-23, the successor to earlier versions of underwater nuclear-capable systems, marked another milestone in Mr. Kim’s program for vastly increasing North Korea’s military arsenal. 

The North linked the test of the underwater attack drone off its east coast to exercises conducted by the American aircraft carrier USS Carl Vinson and South Korean and Japanese ships around the South Korean island of Jeju, 60 miles off the peninsula’s south coast. 

The exercises by “military gangsters,” North Korea’s press agency said, “constituted a cause of further destabilizing the regional situation, and they are an act of seriously threatening the security of the DPRK” — the Democratic People’s Republic of Korea. 

Simultaneously, the North’s press agency made much of a three-day visit to Moscow by North Korea’s foreign minister, Choe Son-hui, in which she met President Putin and Russia’s foreign minister, Sergei Lavrov, in “a show of deepening bilateral ties.” 

North Korea, supplying Russia with artillery shells and arms for its war in Ukraine, counts on Russia for assistance on weapons ranging from nukes to spy satellites to missiles, both intercontinental and sub-surface.

The communist press agency left no doubt of the importance of the meeting in a dispatch just preceding its report on the underwater drone. 

The talk between Ms. Choe and her Russian hosts “reaffirmed the stand of both sides to promote the dynamic development of the overall bilateral relations,” said the North’s press agency, hailing “a new heyday of the strategic and traditional DPRK-Russia friendship” as they “closely cooperate and keep pace with each other for ensuring the regional and global peace and stability.”

The wording of the North’s dispatch, on the underwater test, was deliberately vague. The device was “under development,” it said. “Our army’s underwater nuke-based countering posture is being further rounded off”  while “its maritime and underwater responsive actions continue to deter the hostile military maneuvers of the navies of the United States and its allies.”

South Korea’s military command came up with an equally strong response, declaring that “under a robust South Korea-U.S. combined defense posture” the South would “respond overwhelmingly if the North stages a direct provocation against South Korea.”

It was the connection between Pyongyang and Moscow that caused most concern. South Korea’s Yonhap News quoted the director for arms control at the National Security Council, Pranay Vaddi, as perceiving “an unprecedented level of cooperation in the military sphere” between Russia and North Korea.

“We have never seen this before,” Mr. Vaddi was quoted as saying. “The nature of North Korea as a threat in the region could drastically change over the coming decade as a result of this cooperation.”