North Korea threatened on Tuesday to abandon the 1953 armistice that ended the Korean War if the United States goes ahead with alleged plans to impose a naval blockade and other steps as preparation for a pre-emptive attack.

A spokesman of the North's Korean People's Army said that "the situation on the Korean Peninsula is getting extremely tense" because the United States is planning to send reinforcements in a standoff over the North's nuclear activities.

North Korea "will be left with no option but to take a decisive step to abandon its commitment to implement the Armistice Agreement as a signatory to it and free itself from the binding force of all its provisions," said the unidentified spokesman, quoted by the North's state-run KCNA news agency.

The 1950-53 Korean War ended with the armistice, not a peace treaty, leaving the Koreas technically in a state of war. A North Korean withdrawal from the armistice would greatly increase tensions and uncertainty along the world's most heavily armed border.

State Department officials in Washington had no immediate reaction to the North's threat. A U.S. official, speaking on condition of anonymity, noted the Bush administration had said in the past it would not respond to threats or blackmail.

The North, locked in an international dispute over its nuclear weapons development, has often accused the United States of planning an attack against the communist country. KCNA said on Monday that the North would triumph in the nuclear standoff.

Pyongyang's provocations are widely considered attempts to pressure Washington into direct negotiations with the reclusive regime. The impoverished North is desperate for food and energy aid, and has demanded a nonaggression pact with the United States.

A spokesman of the South Korean Defense Ministry, speaking on condition of anonymity, said on Tuesday that there were no unusual movements by North Korean troops along the 150-mile border.

The statement came as the U.S. military announced on Monday that it would conduct two joint military exercises with South Korea next month, though it said the annual maneuvers are not related to the nuclear dispute with North Korea. Washington keeps 37,000 U.S. troops in South Korea.

The North's statement on the armistice did not specifically refer to the exercises, but Pyongyang has routinely denounced past joint U.S.-South Korea action as preparations for an invasion.

The North Korean armistice statement was issued Monday by the spokesman of the North Korean military's mission to Panmunjom, a truce village where the U.S.-led U.N. Command and the North Korean military meet to oversee the armistice. KCNA did not release the statement until Tuesday morning.

The U.N. Command, which supervises the southern half of the Demilitarized Zone between the two Koreas, had no immediate comment on the North Korean statement.

The North Korean spokesman accused the United States of violating the armistice agreement by sending reinforcements including aircraft carriers and "strategic bombers" around the Korean Peninsula.

The statement also protested the briefly interception of a North Korean cargo ship carrying Scud missiles to Yemen in December, and accused the United States of planning a naval blockade.

"If the U.S. side continues violating and misusing the armistice agreement as it pleases, there will be no need for the (North) to remain bound to the armistice agreement uncomfortably," the spokesman said.

The North has long been chipping away at the armistice, calling it a "useless scrap of paper." It has refused to participate in armistice commission talks, and expelled envoys from China, Poland and the Czech Republic sent to help enforce the armistice on its side of the border.

Last week, the Vienna-based International Atomic Energy Agency referred the North Korean nuclear dispute to the U.N. Security Council. The council could consider economic sanctions against North Korea. The North has said it would consider any sanctions as a declaration of war.

The standoff began in October when U.S. officials said North Korea admitted having a covert nuclear program. Washington and its allies suspended fuel shipments, and the North retaliated by expelling U.N. monitors, taking steps to restart frozen nuclear facilities and withdrawing from the Nuclear Nonproliferation Treaty.