North Korea said Friday it has tested a nuclear-capable underwater attack drone in response to a combined naval exercise by South Korea, the United States and Japan this week, as it continues to blame its rivals for raising tensions in the region.

The test of the drone, purportedly designed to destroy naval vessels and ports, came days after North Korean leader Kim Jong Un declared he is scrapping his country’s long-standing goal of a peaceful reunification with South Korea and that his country will rewrite its constitution to define South Korea as its most hostile foreign adversary.

Tensions on the Korean Peninsula have risen to their highest point in years, with Kim accelerating his weapons testing and threatening nuclear conflict. The United States and its Asian allies have responded by strengthening their combined military exercises, which Kim calls rehearsals for an invasion.

The underwater drone, which North Korea said it first tested last year, is among a broad range of weapon systems demonstrated in recent years as Kim expands his arsenal of nuclear-capable weapons. South Korea’s military says North Korea has exaggerated the capabilities of the drone.

North Korea’s military said it conducted the test in the country’s eastern waters in response to a naval drill by the U.S., South Korea and Japan which ended Wednesday in waters south of Jeju island. It did not say when the test occurred.

“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,” North Korea’s Defense Ministry said in a statement.

“We strongly denounce the U.S. and its followers for their reckless acts of seriously threatening the security of (North Korea) from the outset of the year and sternly warn them of the catastrophic consequences to be entailed by them,” it said.

South Korea’s Defense Ministry denounced North Korea’s recent tests as a violation of U.N. Security Council resolutions and a threat to “peace in the Korean Peninsula and the world.” It said in a statement that the U.S. and South Korean militaries were maintaining a firm defense posture against possible North Korean provocations.

U.N. Secretary-General Antonio Guterres is very concerned about the current situation on the Korean peninsula, U.N. spokesman Stephane Dujarric said. Guterres reiterated his call “for de-escalation, the full implementation of Security Council resolutions and for the creation of an environment that's conducive to dialogue and the resumption of diplomatic talks which are frankly the only possible path forward,” Dujarric said.

North Korea in recent months has tested various missile systems designed to target the United States and its Asian allies, and announced an escalatory nuclear doctrine that authorizes the military to conduct preemptive nuclear strikes if North Korea's leadership is under threat.

North Korea conducted its first ballistic missile test of 2024 on Sunday. State media described it as a new solid-fuel, intermediate-range missile tipped with a hypersonic warhead, likely intended to target U.S. military bases in Guam and Japan.

At an emergency meeting of the U.N. Security Council on Thursday, South Korea urged the council “to break the silence” over North Korea’s escalating missile tests and threats. Russia and China, both permanent members of the Security Council, have blocked U.S.-led efforts to increase sanctions on North Korea over its recent weapons tests, underscoring a divide deepened over Russia’s war on Ukraine. South Korea is serving a two-year term on the council.