SEOUL — North Korea lashed out at the United States and South Korea on Wednesday, warning that it would attack the South if any of its ships were intercepted as part of an American-led initiative to stem the world trade in nuclear weapons.
North Korea's state-run news service reiterated Pyongyang's anger over South Korea's decision to join 100 other nations in the so-called Proliferation Security Initiative to blockade any nation suspected of trading nuclear materials.
"The U.S.-led PSI is now inching close to an extreme phase where a war may break out any moment," said the Korean Central News Agency, accusing South Korea of "blindly yielding to its master as it is steeped in sycophancy and submission to the marrow of its bones."
The development came amid reports that North Korea had staged mass rallies to celebrate its second nuclear test, an event Monday that has brought condemnation worldwide.
The North Korean news service, which is often used by leader Kim Jong Il to express policy statements, added that it considered Seoul's move to join the Proliferation Security Initiative a "declaration of war."
"Any hostile act against our peaceful vessels, including search and seizure, will be considered an unpardonable infringement on our sovereignty and we will immediately respond with a powerful military strike," the news service said, citing a spokesman for the North Korean army.
U.S. security studies have identified North Korea as a supplier of missiles and nuclear materials to nations such as Syria and Pakistan in exchange for cash.
In Washington, the White House dismissed the North Korean threats as "bluster."
"Threats won't get North Korea the attention it craves," White House spokesman Robert Gibbs said.
Also Wednesday, experts were looking for signs that Pyongyang may have begun reprocessing spent nuclear fuel rods to harvest weapons-grade plutonium. A Seoul newspaper reported that U.S. spy satellites detected steam coming from North Korea's nuclear facility in Yongbyon. The Washington-based Institute for Science and International Security said a review of intelligence photos showed that the mysterious white smoke cited was coming from an adjacent coal-fired plant that generates steam for tasks at the reprocessing plant.
"People are probably out there trying to get some air samples -- from friendly aircraft or ships at sea," said Daniel Pinkston, northeastern Asia deputy project director for the think tank International Crisis Group. "They're looking to see if any elements are released into the air when fuel rods are cut open."
North Korea had stopped reprocessing fuel rods in an agreement struck in 2007. But in recent months there have been numerous setbacks to the international community's attempt to disarm the secretive state.
Last month, North Korea launched a rocket in what many think was a long-range-missile test. Pyongyang said it was a peaceful satellite launch, though Western intelligence reported no sign that a satellite had been released into orbit.
After the U.S., South Korea and Japan sought to punish Pyongyang for the launch through new sanctions from the U.N. Security Council, North Korea withdrew from the so-called six-party denuclearization talks, which also involved China and Russia.
Anxiety increased this week with North Korea's nuclear test, which was followed by what South Korean officials confirmed was the launch of five short-range missiles.
North Korea also announced Wednesday that it would no longer be bound by the armistice that ended hostilities in the Korean War. The move caused many analysts to speculate that North and South Korea may be reentering a Cold War-like stalemate.
"This is not good. It's disturbing and troubling," Pinkston said. "But I'll take a cold war over general full-scale war any day."
Other analysts speculated that the chance for some kind of armed conflict between the two sides rose daily.
Chances for provocation from Pyongyang "are higher than before," said Lee Woo-young, a professor at the University of North Korean Studies in Seoul.
"They will not easily abandon nuclear weapons," Lee said. "For now, North Korea will await any reaction from the U.S. before taking the next step. But in the short run, it will be hard to move any negotiations forward."
North Korea said Wednesday that it would no longer guarantee the safety of U.S. and South Korean ships off its southwestern coast. But experts have mostly ruled out a naval conflict.
"North Korean naval power is weaker than South Korea's," said Cheong Seong-chang, director of the Inter-Korean Relations Studies Program at the Sejong Institute. "So North Korea will act carefully."
North Korea's government-run news service indicated that thousands of citizens joined senior military and party officials Tuesday at a stadium in Pyongyang to celebrate the nuclear test.
"The North's leadership is looking at a complicated chessboard; they have to address a number of problems, including being the weakest military in the region," Pinkston said. "They believe, despite the cost they have to bear, they are going to move forward with their nuclear power to deter external foes and pursue science.
"You can sense the growing anger in their news service declarations," he said. "They are very angry at the world. They think that everybody is picking on them."
Times staff writer Paul Richter in Washington and Ju-min Park of The Times' Seoul Bureau contributed to this report.