In a move certain to escalate tensions with the United States, North Korea ordered U.N.-certified nuclear inspectors to leave the country Friday and announced it will reactivate an atomic lab at which it can produce weapons-grade plutonium.
The U.S. has said the facility can produce enough plutonium to make several atomic bombs.
The U.N. inspectors were the last means that the International Atomic Energy Agency had to monitor whether North Korean nuclear facilities, shut down in a deal with the U.S. in 1994, will be used for weapons projects when they are reactivated.
Despite IAEA warnings, the North Korean government removed monitoring seals and surveillance cameras from the nuclear complex at Yongbyon earlier this week.
The United States later in the day reiterated that it continues to seek a peaceful resolution of the issue, but it "will not negotiate in response to threats or broken commitments."
"We call on the DPRK (North Korea) to reverse its current course and to take all steps necessary to comply with the IAEA ... to eliminate its nuclear weapons program in a verifiable manner," White House spokeswoman Claire Buchan said.
Amid the rising tension, the U.S.-U.N. Command said Friday that the North had violated the armistice that ended the Korean War by bringing machine guns into the buffer zone separating the two Koreas six times in the past two weeks.
The command said that in each instance, the North Korean troops removed the weapons from the Demilitarized Zone at the end of each day. Under the 1953 armistice, only rifles and other small arms are allowed inside.
North Korea sent letters to Mohamed El-Baradei, head of the Vienna-based IAEA, demanding the removal of the inspectors and announcing the reopening of the reprocessing lab, the North's official news agency KCNA said Friday.
Pyongyang said it was reopening the lab to give "safe storage" to spent fuel rods that will come from the reactor it plans to restart.
The lab can be used to extract weapons grade plutonium from spent fuel rods. North Korea already has 8,000 spent fuel rods in storage that experts say could yield four or five nuclear weapons within months. The KCNA statement, monitored by South Korea's Yonhap news agency, made no mention of those stockpiled rods.
The IAEA did not comment on the reported announcement on the lab, but a spokesman confirmed the agency received "a letter saying take out the inspectors."
The North's letter did not clarify when it will expel the inspectors, Yonhap said.
"Our government has decided to expel the IAEA inspectors because there is no reason that they should stay in our country," Ri Je Son, the North's chief nuclear policy-maker, was quoted as saying in the KCNA report.
Meanwhile, the IAEA said the North was moving fresh fuel cells into the 5-kilowatt reactor at Yongbyon. By Friday, about 2,000 new rods had been moved to a storage facility at the site, up from 1,000 a day earlier, IAEA spokeswoman Melissa Fleming said.
The reactor needs 8,000 rods to be started, Fleming said.
North Korea claims that it is restarting the reactor to generate badly needed electricity after the United States and its allies cut off oil shipments.
The oil supplies were cut off in response to recent revelations that the North Koreans had been covertly pressing ahead with efforts to develop nuclear weapons in violation of the 1994 agreement.
But U.S. officials say that power to be obtained from the 5-megawatt reactor is negligible, and North Korea is widely believed to be pushing the dispute to the brink of crisis in order to extract concessions at the negotiating table.
South Korea on Friday convened an emergency meeting of its National Security Council to discuss the North's announcement.
Earlier, South Korea's President-elect Roh Moo-hyun said North Korea's defiant attitude could make it difficult for him to continue his predecessor's policy of seeking reconciliation with Pyongyang after he takes office in February.
"Whatever North Korea's rationale is in taking such actions, they are not beneficial to peace and stability on the Korean Peninsula and in Northeast Asia, nor are they helpful for its own safety and prosperity," Roh said in a statement.
North Korea's government has repeatedly called for a nonaggression treaty with the United States, though economic benefits are also a priority for the destitute country.
The North's state media accused Washington on Friday of using the nuclear issue as a pretext for invasion. An English-language commentary by the KCNA referred to recent comments by Secretary of Defense Donald Rumsfeld asserting that the United States is capable of waging war against Iraq and North Korea at the same time.
"The U.S. much-publicized assertion that North Korea scrap its nuclear program first is nothing but a pipe-dream as it calls for disarming (North Korea) under the absurd pretext of its nuclear program and then launching a surprise attack on it to overthrow its political system," KCNA said.
During an inspection tour of U.S. and South Korean air force units, outgoing President Kim Dae-jung called for a stronger military alliance between the two allies to cope with threats raised by North Korea's nuclear development.
"We should be fully prepared for any emergencies and maintain a tighter joint defense system to back up a peaceful solution to North Korea's nuclear issue," he said.
The Associated Press contributed to this report.