SEOUL, South Korea - North Korea responded to new U.N. sanctions with more defiance, promising yesterday to step up its nuclear bomb-making program by enriching uranium and threatening war on any country that dares to stop its ships.

The North's threats were the first public acknowledgment that the reclusive communist nation has been running a secret uranium-enrichment program. Suspicions of the program touched off the latest nuclear crisis in 2002.

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 United States 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 United States 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 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.

Secretary of State Hillary Rodham Clinton said the new U.N. penalties provided 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 yesterday 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, North Korea also said it had 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 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 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 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 was engaging in a game of "chicken" with the United States that he predicted would eventually end in bilateral talks.