The country also vowed never to give up its nuclear ambitions as a way to protect its sovereignty amid signs of preparations for naming its ailing leader Kim Jong Il's youngest son, Jong Un, as his successor.
Despite repeated assurances from Washington, North Korea has harbored deep-rooted suspicions that the U.S. could invade to topple its regime.
"It has become an absolutely impossible option for (North Korea) to even think about giving up its nuclear weapons," the North's Foreign Ministry said in a statement carried by the country's official Korean Central News Agency.
North Korea also warned that any attempted blockade by the U.S. and its allies would be regarded as "an act of war and met with a decisive military response."
The new threats came in response to tough new sanctions imposed by the U.N. Security Council over the North's second nuclear test on May 25.
The sanctions are aimed at depriving North Korea of the financing used to build its rogue nuclear program. The resolution also authorized searches of North Korean ships suspected of transporting illicit ballistic missile and nuclear materials.
U.S. Secretary of State Hillary Rodham Clinton said the new U.N. penalties provide the necessary tools to help check North Korea's continued pursuit of nuclear weapons.
"This was a tremendous statement on behalf of the world community that North Korea's pursuit of nuclear weapons and the capacity to deliver those weapons through missiles is not going to be accepted by the neighbors as well as the greater international community," Clinton said Saturday at a news conference in Niagara Falls, Ontario.
"I think these sanctions ... give the world community the tools we need to take appropriate action."
In a move that could further escalate the nuclear standoff with the U.S., North Korea also said it has reprocessed more than a third of its spent nuclear fuel rods and vowed to weaponize its new plutonium, a key ingredient of atomic bombs along with enriched uranium.
North Korea is believed to have about 110 pounds (50 kilograms) of plutonium, enough for a half dozen bombs, said Yoon Deok-min, a professor at South Korea's state-run Institute of Foreign Affairs and National Security.
Reprocessing 8,000 spent nuclear fuel rods stored at North Korea's Yongbyon nuclear complex could yield an additional 18 to 22 pounds (8-10 kilograms) of plutonium - enough to make at least one more atomic bomb, he said.
The North's announcement represents a huge setback for an aid-for-disarmament deal aimed at ending its nuclear ambitions, and presents a new diplomatic headache for President Barack Obama as he prepares for talks with his South Korean counterpart on Tuesday on the North's missile and nuclear issues.
Analyst Kim Yong-hyun of Seoul's Dongguk University said North Korea was sending a stern message to Washington ahead of the meeting.
He said North Korea is engaging in a game of "chicken" with the U.S. that he predicted would eventually end in bilateral talks. South Korea expressed serious concern and regret over the North's statement.
"The provocative steps can never be tolerated," the South's Foreign Ministry said in a statement, urging the North to return to stalled disarmament talks.
In a February 2007 deal, North Korea agreed to begin disabling Yongbyon in return for the equivalent of 1 million tons of fuel oil and other concessions from the U.S., South Korea, Russia, China and Japan.
But disablement came to a halt as North Korea wrangled with Washington over how to verify its past atomic activities. The latest round of talks, in December, failed to make any progress. North Korea has said it will test a long-range missile and is suspected of preparing for a third nuclear test, but there is no evidence that either is imminent.
Meanwhile, South Korea's top admiral expressed a firm intention to fight back against any North Korean provocations along their disputed western sea border, saying another deadly naval skirmish could occur in the area, where there were deadly naval clashes in 1999 and 2002.
As a precaution, South Korea has dispatched hundreds more marines to two islands near the maritime border.
"Be ready to chop off the wrist of the enemy if it touches even the tip of our hand," navy Chief of Staff Jung Ok-keun said in the text of a speech to be read at a Monday ceremony marking the 10th anniversary of the 1999 naval battle.
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Associated Press writers Edith M. Lederer at the United Nations and Carolyn Thompson in Niagra Falls, Ontario, contributed to this report.
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