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

North Korea’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.

“It has become an absolutely impossible option for (North Korea) to even think about giving up its nuclear weapons,” North Korea’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 North Korea’s second nuclear test on May 25.

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.

North Korea’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 North Korea’s missile and nuclear issues.

Analyst Kim Yong-hyun of Seoul’s Dongguk University 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 North Korea’s statement.

“The provocative steps can never be tolerated,” South Korea’s Foreign Ministry said in a statement, urging North Korea 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.