What Is HAEIL-5-23? North Korea's Underwater Nuke System Test Raises Alarm

North Korea has carried out a test of an "underwater nuclear weapons system," the secretive nation's state media reported on Friday, in the latest escalation of tensions in the Indo-Pacific as Pyongyang forges ahead with weapons testing.

Pyongyang has tested its "Haeil-5-23" in the East Sea, also known as the Sea of Japan, North Korea's Defense Ministry said on Friday, according to the state-run Korean Central News Agency (KCNA). It was initially tested in early 2023, described by press releases from the country as an "underwater nuclear attack drone."

This week's test was a response to joint naval drills carried out by the U.S., South Korea and Japan earlier this week, Pyongyang said. The large-scale, three-day drills involved the USS Carl Vinson carrier strike group, and "enhance combined readiness and capabilities and support a free and open Indo-Pacific," the U.S. military said.

The military exercises "constituted a cause of further destabilizing the regional situation," and are considered a serious threat to North Korea's national security, Pyongyang said.

"We strongly denounce the U.S. and its followers for their reckless acts of seriously threatening the security of the DPRK [North Korea] from the outset of the year and sternly warn them of the catastrophic consequences to be entailed by them," North Korea's Defense Ministry said in a statement.

North Korea has ramped up its munitions industry and nuclear weapons sector, and carried out a slew of recent ballistic missile tests that go against United Nations sanctions.

North Korea said in March 2023 that it had tested the "Haeil" for several days before detonating the test warhead. "Haeil," or "tsunami," is designed to carry out stealth attacks in enemy waters, state media reported.

"The reliability and safety of the weapon system were verified," KCNA reported at the time. A report published by KCNA weeks later said Pyongyang had tested another underwater nuclear attack drone for 71 hours and proved its "fatal attack ability."

South Korean officials have previously said the drone's capabilities are exaggerated.

In September, North Korea said it had debuted its first "tactical nuclear attack submarine," able to carry and launch nuclear weapons. There is some doubt among Western analysts about the submarine's true capabilities.

In mid-December, Pyongyang fired a long-range ballistic missile in a test that put the "whole of the U.S. territory" in North Korea's range, Japan said at the time. Pyongyang fired another ballistic missile on Sunday, which the U.S. military said "highlights the destabilizing impact of the DPRK's illicit weapons program."

In late November, North Korea launched its first military reconnaissance satellite, which the North Korean leader heralded as a "new era of space power." The United Nations slammed the launch as using banned ballistic missile technology.