The United States Thursday declared that it was "fully prepared" to deal with any kind of provocation against the United States after Pyongyang threatened to attack the US, DPA reported.
"We are very concerned with North Korea's continuing provocative behavior," Secretary of Defence Leon Panetta said.
"If they go forward with the kind of tests that they're talking about, it again represents a violation of the UN security resolutions and a violation of international law," he said. "We are fully prepared ... to deal with any kind of provocation from the North Koreans."
Earlier in the day, North Korea's National Defence Commission said it would open a "new phase" of its "century-long struggle against the United States" in which it would no longer hide "the fact that various satellites; long-range missiles that we will continue to launch; and a high-level nuclear test ... will target our sworn enemy, the United States."
Experts said Pyongyang's preparations for a third nuclear test were far enough along that it could happen at any time.
North Korea's statement was issued after the UN Security Council voted unanimously Tuesday to impose additional sanctions on one of the world's most highly sanctioned countries after Pyongyang successfully launched a long-range rocket last month.
White House spokesman Jay Carney said the threats will "only increase Pyongyang's isolation."
"We judge North Korea by its actions, and provocations like these are significant violations," Carney said. "And we act accordingly."
In Brussels, European Union foreign policy chief Catherine Ashton called on North Korean authorities to "choose a more constructive path through engagement with the international community, including in the framework of the six-party talks."
"I deplore the announcement by the North Korean authorities that they intend to proceed with further provocative actions, including a new nuclear test," she said.
In Seoul, a spokesman for South Korea's Defence Ministry said: "It is our understanding that if the leadership gives consent, the North can detonate a nuclear device whenever it wants to."
Pyongang gave no dates for when a test or launch might be carried out, but intelligence, including satellite photos, has indicated for months that North Korea was making preparations at its nuclear test site. Experts in the US also said the preparations were now far enough along that another test could occur at any time.
A series of sanctions have been imposed on Pyongyang for its first nuclear tests in 2006 and 2009 and its rocket launches. They have failed to put a stop to them.
The North Korea Defence Commission called all the UN sanctions unlawful and illegitimate and charged that they were led by the US and represented "the most dangerous phase of the hostile policy" towards Pyongyang.
"Our peaceful satellites will continue to rise up without any disruption amid our national struggle to defend our right to self-defence," said the commission, which is the top decision-making body in a communist country that has one of the world's largest militaries and is one of the globe's poorest nations.
Immediately after the UN sanctions action, North Korea said it would no longer participate in the already stalled talks on the denuclearization of the Korean Peninsula because of Washington's "worsening policy of hostility towards North Korea."
If a third nuclear test is carried out, it would be the first under the leadership of Kim Jong Un, who took over the rule of North Korea in December 2011 as the third member of the Kim dynasty to do so.
The United States sided with South Korea against the North in the 1950-53 Korean War, and Washington's and Pyongyang's relations have been marked by antagonism ever since. They have never established formal diplomatic ties.