SEOUL, South Korea -- North Korea's top governing body warned Thursday that the regime will conduct its third nuclear test in defiance of UN punishment, and made clear that its long-range rockets are designed to carry not only satellites but also warheads aimed at striking the United States.

The National Defence Commission, headed by the country's young leader, Kim Jong Un, denounced Tuesday's UN Security Council resolution condemning North Korea's long-range rocket launch in December as a banned missile activity and expanding sanctions against the regime. The commission reaffirmed in its declaration that the launch was a peaceful bid to send a satellite into space, but also clearly indicated the country's rocket launches have a military purpose: to strike and attack the United States.

While experts say North Korea doesn't have the capability to hit the U.S. with its missiles, recent tests and rhetoric indicate the country is feverishly working toward that goal.

The commission pledged to keep launching satellites and rockets and to conduct a nuclear test as part of a "new phase" of combat with the United States, which it blames for leading the UN bid to punish Pyongyang. It said a nuclear test was part of "upcoming" action but did not say exactly when or where it would take place.

"We do not hide that a variety of satellites and long-range rockets which will be launched by the DPRK one after another and a nuclear test of higher level which will be carried out by it in the upcoming all-out action, a new phase of the anti-U.S. struggle that has lasted century after century, will target against the U.S., the sworn enemy of the Korean people," the commission said, referring to North Korea by its official name, the Democratic People's Republic of Korea.

"Settling accounts with the U.S. needs to be done with force, not with words, as it regards jungle law as the rule of its survival," the commission said.

It was a rare declaration by the powerful commission once led by late leader Kim Jong Il and now commanded by his son. The statement made clear Kim Jong Un's commitment to continue developing the country's nuclear and missile programs in defiance of the Security Council, even at risk of further international isolation.

North Korea's allusion to a "higher level" nuclear test most likely refers to a device made from highly enriched uranium, which is easier to miniaturize than the plutonium bombs it tested in 2006 and 2009, said Cheong Seong-chang, an analyst at the private Sejong Institute in South Korea. Experts say the North Koreans must conduct further tests of its atomic devices and master the technique for making them smaller before they can be mounted as nuclear warheads onto long-range missiles.

The U.S. State Department had no immediate response to Thursday's statement. Shortly before the commission issued its declaration, U.S. envoy on North Korea Glyn Davies urged Pyongyang not to explode an atomic device.

"Whether North Korea tests or not, it's up to North Korea. We hope they don't do it. We call on them not to do it," he told reporters in Seoul after meeting with South Korean officials. "It will be a mistake and a missed opportunity if they were to do it."

Davies was in Seoul on a trip that includes his stops in China and Japan for talks on how to move forward on North Korea relations.

South Korea's top official on relations with the North said Pyongyang's nuclear and missile development is a "cataclysm for the Korean people," and poses a fundamental threat to regional and world peace. "The North Korean behaviour is very disappointing," Unification Minister Yu Woo-ik said in a lecture in Seoul, according to his office.

North Korea claims the right to build nuclear weapons as a defence against the United States, its Korean War foe.

The bitter three-year war ended in a truce, not a peace treaty, in 1953, and left the Korean Peninsula divided by the world's most heavily fortified demilitarized zone. The U.S. leads the UN Command that governs the truce and stations more than 28,000 troops in ally South Korea, a presence that North Korea cites as a key reason for its drive to build nuclear weapons.

For years, North Korea's neighbours had been negotiating with Pyongyang on providing aid in return for disarmament. North Korea walked away from those talks in 2009 and on Wednesday reiterated that disarmament talks were out of the question.

North Korea is estimated to have stored up enough weaponized plutonium for four to eight bombs, according to scientist Siegfried Hecker, who visited the North's Nyongbyon nuclear complex in 2010.

In 2009, Pyongyang declared that it would begin enriching uranium, which would give North Korea a second way to make atomic weapons.

North Korea carried out underground nuclear tests in 2006 and 2009, both times just weeks after being punished with UN sanctions for launching long-range rockets.

In October, an unidentified spokesman at the National Defence Commission claimed that the U.S. mainland was within missile range. And at a military parade last April, North Korea showed off what appeared to be an intercontinental ballistic missile.

Satellite photos taken last month at a nuclear test site in Punggye-ri, in far northeast North Korea, showed continued activity that suggested a state of readiness even in winter, according to analysis by 38 North, a North Korea website affiliated with the Johns Hopkins School for Advanced International Studies.

Another nuclear test would bring North Korea a step closer to being able to launch a long-range missile tipped with a nuclear warhead, said Daniel Pinkston, an analyst with the International Crisis Group.

"Their behaviour indicates they want to acquire those capabilities," he said. "The ultimate goal is to have a robust nuclear deterrent."