A Foreign Ministry spokesman said the North had built atomic weapons to counter the U.S. nuclear threat.
"As we declared, our strong revolutionary might put in place all measures to counter possible U.S. pre-emptive strike," the spokesman said, according to the Korean Central News Agency. "Pre-emptive strike is not the monopoly of the United States."
The United States urged North Korea to return to international nuclear negotiations instead of making inflammatory statements. State Department spokesman Sean McCormack said the United States has no plans to invade or attack North Korea.
Last week, the communist country warned that it had the right to launch a pre-emptive strike, saying it would strengthen its war footing before joint South Korea-U.S. military exercises scheduled for this weekend.
The North's spokesman said it would be a "wise" step for the United States to cooperate on nuclear issues with North Korea in the same way it does with India.
Earlier this month, President Bush signed an accord in India that would open some of its atomic reactors to international inspections in exchange for U.S. nuclear know-how and atomic fuel.
The accord was reached even though New Delhi has not signed the international Nuclear Nonproliferation Treaty. North Korea has withdrawn from the treaty and condemned the United States for giving India "preferential" treatment.
"If the U.S. is truly interested in finding a realistic way of resolving the Korean Peninsula nuclear issue, it would be wise for it to come out on the path of nuclear cooperation with us," the North Korean spokesman said.
The North's announcement that it has a nuclear arsenal risked escalating tensions in the prolonged standoff over its program and threatened the prospect of resuming six-nation talks on the dispute.
"We have built nuclear weapons for no other purpose than to counter U.S. nuclear threats," the Foreign Ministry spokesman said.
It is rare for North Korea to mention its nuclear capabilities in such an explicit manner. The communist state usually refers to its "nuclear deterrent force."
North Korea first declared last year that it has nuclear weapons, although the claim could not be confirmed independently. Experts believe the North has extracted enough plutonium from its main nuclear reactor for at least a half-dozen weapons.
Six-nation talks have been stalled since November over a dispute surrounding financial restrictions the United States imposed on North Korea for its alleged currency counterfeiting and money laundering. Those talks involve the two Koreas, the United States, China, Japan and Russia.
Pyongyang says it will not return to the negotiating table unless the restrictions are lifted. But Washington demands that the North come to the talks without preconditions, saying the two issues are separate.
The North's spokesman said his country had shown "maximum flexibility" in trying to resolve the financial dispute, proposing possible solutions during a meeting in New York earlier this month. The meeting produced no breakthrough.
"The Bush administration talks about six-party talks, but it actually is paying no attention to the talks," the spokesman said, according to KCNA.
McCormack said South Korea's new nuclear envoy, Chun Young-woo, will meet later this week with top State Department officials. No date has been set for a resumption of the nuclear talks, McCormack said.
The North Korean spokesman also disputed last week's U.S. national security report that, among other things, said North Korea posed a serious nuclear proliferation challenge.
"In a word, it is a robbery-like declaration of war," the spokesman said. "Through this document, the Bush administration declared to the world that it is a group of war fanatics."