SEOUL (Reuters) - North Korean leader Kim Jong Un said his country would soon test a nuclear warhead and ballistic missiles capable of carrying nuclear warheads, North Korea’s KCNA news agency reported on Tuesday, in what would be a direct violation of U.N. resolutions that have the backing of Pyongyang’s chief ally, China.

Kim made the comments as he supervised what KCNA said was a successful simulated test of atmospheric re-entry of a ballistic missile that measured the “thermodynamic structural stability of newly developed heat-resisting materials”.

“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’s defense ministry said there were no indications of activities at the North’s nuclear test site or its long-range rocket station, but that North Korea continued to maintain readiness to conduct nuclear tests.

The North Korean report comes amid heightened tension on the Korean peninsula after the announcement of new U.N. sanctions on North Korea and as South Korean and U.S. troops stage their largest ever annual joint military exercises.

South Korean President Park Geun-hye said North Korea would lead itself to self-destruction if it continued its confrontation with the international community.

Pentagon spokesman Peter Cook condemned the North Korean rhetoric, while questioning Pyongyang’s technological claims.

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“We have not seen North Korea demonstrate capability to miniaturize a nuclear weapon, and again, put it on a ballistic missile,” he told a news briefing.

China, which has been North Korea’s main ally but backed the tougher U.N. sanctions, urged caution and called for all sides to avoid actions that would exacerbate tensions.

But in a sign of Beijing’s frustration with Pyongyang, China’s state news agency Xinhua said that the South Korean and Chinese foreign ministers had discussed the sanctions by telephone late on Monday and agreed on the importance of implementing them “in a complete and comprehensive manner”.

South Korea’s defense ministry said that despite the KCNA report, it did not believe North Korea had acquired the re-entry technology needed to prevent ballistic missiles burning up when they reenter the earth’s atmosphere.

NORTH KOREAN TESTS

In the apparent re-entry simulation, the official newspaper of North Korea’s ruling Workers’ Party showed pictures of a dome-shaped object placed under what appeared to be a rocket engine being blasted with flaming exhaust. In separate images, Kim observed the object described by KCNA as a warhead tip.

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North Korea has issued belligerent statements almost daily since coming under a new U.N. sanctions resolution this month after it carried out its fourth nuclear test in January and launched a long-range rocket last month.

Nuclear proliferation expert Jeffrey Lewis of the California-based Middlebury Institute of International Studies said it would be a “terrible” worry if North Korea was seeking to emulate U.S. and Chinese tests in 1962 and 1966 respectively that launched ballistic missiles with live warheads.

“For now, though, it looks like a nuclear test and several missile tests in close succession,” he said.

Kim said last week his country had miniaturized a nuclear warhead.

North Korea also claims that its January nuclear test was of a hydrogen bomb, although most experts said the blast was too small for it to have been from a full-fledged hydrogen bomb.

North Korea rejects criticism of its nuclear and missile programmers, even from China, saying it has a sovereign right to defend itself from threats.

The U.N. Security Council resolution sharply expanded existing sanctions by requiring member states to inspect all cargo to and from North Korea and banning North Korea’s trade of coal when it is seen as funding its arms programs.