North Korean leader Kim Jong Un has ordered an expansion of the country's production of weapons-grade nuclear materials to accelerate its output of powerful nuclear weapons, state-run media said Tuesday.

Kim gave the instructions on Monday as he inspected work being done for mounting nuclear warheads on ballistic missiles, according to the official Korean Central News Agency, which released pictures of objects believed to be warheads of different types.

The leader's order came as the United States and South Korea conduct joint military exercises. On Tuesday, the U.S. nuclear-powered aircraft carrier Nimitz arrived at a port in the southern South Korean city of Busan. North Korea has denounced such exercises as a rehearsal for invasion.

North Korean leader Kim Jong Un (C) is pictured at an unspecified location as he inspects work for mounting nuclear warheads on ballistic missiles on March 27, 2023. (Parts of photo pixelated by KCNA)(KCNA/Kyodo)

Pyongyang has launched a series of ballistic missiles recently, with fears mounting that the country may be preparing to conduct its seventh nuclear test.

Kim was briefed on details of the nuclear program such as "the means for applying nuclear weapons" and technological specifications and examined the country's plan and written orders for nuclear counterattack operations, KCNA said.

The Rodong Sinmun, the official newspaper of the Workers' Party of Korea, criticized Tuesday the U.S.-South Korea joint drill and insisted it is "natural that our nuclear force will go on an important mission when the country's autonomy and security is threatened."

KCNA also said a unit guided by the Missile General Bureau of the North's armed forces on Monday test-fired two ground-to-ground tactical ballistic missiles loaded with mock nuclear warheads, with both detonating 500 meters above their intended target.

Japan and South Korea detected the Monday launch of two ballistic missiles off the North's east coast, with the missiles believed to have fallen in the sea outside of Tokyo's exclusive economic zone.

In addition, the North's Academy of Defense Science conducted another test of an underwater nuclear attack drone for three days through Monday, the agency said. Pyongyang also tested the new system last week, claiming it can generate a radioactive tsunami to destroy enemy ships and ports.

The drone deployed for the test Saturday in Wonsan Bay in the eastern part of North Korea "correctly set off" a test warhead underwater on Monday after cruising along a "jagged and oval course" simulating a distance of 600 kilometers in the Sea of Japan for 41 hours and 27 minutes, KCNA said.

The South Korean military said Monday it believes the North's claims to have developed the new unmanned underwater nuclear attack system are "exaggerated."

Related coverage:

North Korea fires 2 short-range ballistic missiles: South Korean military

New Chinese envoy arrives in North Korea after 15-month COVID hiatus

Japan, South Korea mull 1st security talks in 5 years in April: source