Kim Jong-un says North Korea will soon conduct a nuclear warhead test and test-launch ballistic missiles capable of carrying nuclear warheads.
Such tests, reported in state media on Tuesday, would be in defiant violation of United Nations sanctions that were recently strengthened with the backing of China, North Korea's chief ally.
Kim made the comments as he supervised a successful simulated test of atmospheric re-entry of a ballistic missile that measured the "thermodynamic structural stability of newly developed heat-resisting materials", the North's official KCNA news agency reported.
"Declaring that a nuclear warhead explosion test and a test-fire of several kinds of ballistic rockets able to carry nuclear warheads will be conducted in a short time to further enhance the reliance of nuclear attack capability, he [Kim] instructed the relevant section to make prearrangement for them to the last detail," the agency said.
South Korea doubts claim
In response, South Korea's defence ministry said it did not believe that North Korea had acquired missile re-entry technology.
It also noted there were no indications of activities at the North's nuclear test site, or its long-range rocket station.
But South Korea's defence ministry spokesman Moon Sang-gyun warned: "If North Korea puts this into action, it certainly violates the United Nations Security Council resolution, and it is a grave provocation against the Korean peninsula and the international community.
"We once again urge North Korea to immediately stop this act which leads to its self-destruction."
PHOTOS: South Korea and US forces storm mock North Korea beach
South Korean President Park Geun-hye said the North would lead itself to self-destruction if it continued the confrontation with the international community.
The North's report comes amid heightened tension on the Korean peninsula as South Korean and American troops stage annual military exercises that Seoul has described as the largest ever.
In the apparent re-entry simulation, the official newspaper of North Korea's ruling Workers' Party carried pictures on Tuesday of a dome-shaped object placed under what appeared to be a rocket engine and being blasted with flaming exhaust.
In separate images, Kim observed the object described by KCNA as a warhead tip.
The North has issued belligerent statements almost daily since coming under a new UN resolution adopted this month to tighten sanctions after a nuclear test in January and the launch of a long-range rocket last month.
Rights abuses questioned
In a separate development, the North's human rights record once again came into question as a UN expert on Monday called for leader Kim to be prosecuted for human rights abuses, including starvation and "slave-like conditions" in the country.
"It is now time for the international community to move on with the logical next step - which is pursuing accountability of the regime and those most responsible for violations of human rights in the country," said Marzuki Darusman, the UN's special rapporteur on North Korea's human rights, as he addressed the Human Rights Council in Geneva.
"Nothing happened anywhere in that country without the say-so of the highest supreme leader ... Mr Kim Jong Un."
Speaking to the Reuters news agency, Darusman added: "It may not be a full prosecutorial entity but lays down the initial prosecution process."
Darusman, a former foreign minister of Indonesia, said investigations could be pursued via the International Criminal Court but, failing any consensus among major powers, North Korea's leadership could be prosecuted in a third country.