Secretary of State Hillary Rodham Clinton warned that North Korea will face consequences because of "provocative and belligerent" actions that include threatened military attacks against U.S. and South Korean warships.
Clinton also underscored Wednesday the U.S. commitment to defend South Korea and Japan in the aftermath of North Korea's nuclear and missile tests this week. She said that talks at the United Nations "are going on to add to the consequences that North Korea will face."
Clinton's stern statement came after North Korea threatened military attacks against U.S. and South Korean warships and called Seoul's decision to join an international program to intercept ships suspected of aiding nuclear proliferation tantamount to a declaration of war.
"North Korea has made a choice," Clinton said, to violate U.N. Security Council resolutions, ignore international warnings and abrogate commitments made during six-nation nuclear disarmament talks.
"There are consequences to such actions," Clinton said. She did not provide specifics.
Clinton said she was pleased at a unified international condemnation of North Korea that included Russia and China, North Korea's only major ally and the host of the currently stalled disarmament talks.
The success of any new sanctions would depend on how aggressively China, one of North Korea's only allies, implements them.
Threats and sabre-rattling
Russia's foreign minister said world powers must be firm with North Korea but also take care to avoid inflaming tensions further.
The world "must not rush to punish North Korea just for punishment's sake," Foreign Minister Sergey Lavrov said, adding that Russia wants a U.N. Security Council resolution that will help restart stalled six-nation talks.
North Korea on Monday conducted an underground nuclear test very close to the Russian border. Russia's military said its yield was comparable to the bombs that flattened Hiroshima and Nagasaki, though other estimates have pointed to a smaller blast.
"The Security Council must speak out firmly, and must work out measures that in practice would help prevent the further dissolution of the nuclear nonproliferation regime," Lavrov told reporters. "We must also find in this resolution a way to create conditions for the renewal of the six-nation talks."
At the White House, spokesman Robert Gibbs played down North Korea's angry rhetoric and saber-rattling. He said threats against South Korea will not give North Korea the attention it wants and will only add to its isolation. "My sense is that they're trying to get renewed attention," Gibbs said.
North Korea's latest belligerence comes as the U.N. Security Council debates how to punish the regime for testing a nuclear bomb Monday in what President Barack Obama called a "blatant violation" of international law.
South Korea, divided from the North by a heavily fortified border, had responded to the nuclear test by joining the Proliferation Security Initiative, a U.S.-led network of nations seeking to stop ships from transporting the materials used in nuclear bombs.
Seoul previously resisted joining the PSI in favor of seeking reconciliation with Pyongyang, but pushed those efforts aside Monday after the nuclear test in the northeast.
North Korea warned Wednesday that any attempt to stop, board or inspect its ships would constitute a "grave violation."
'On the raw'
The regime also said it could no longer promise the safety of U.S. and South Korean warships and civilian vessels in the waters near the Korea's western maritime border.
"They should bear in mind that the (North) has tremendous military muscle and its own method of strike able to conquer any targets in its vicinity at one stroke or hit the U.S. on the raw, if necessary," it said.
The maritime border has long been a flashpoint between the two Koreas. North Korea disputes the line unilaterally drawn by the United Nations at the end of the Koreas' three-year war in 1953 and has demanded it be redrawn further south.
The North Korean army called South Korea's actions a violation of the armistice and said it would no longer honor the treaty.
The truce signed in 1953 and subsequent military agreements call for both sides to refrain from warfare, but doesn't cover the waters off the west coast.
North Korea has used the maritime border dispute to provoke two deadly naval skirmishes — in 1999 and 2002.
On Wednesday, the regime promised "unimaginable and merciless punishment" for anyone daring to challenge its ships.
Restarting nuke plant?
Pyongyang also reportedly restarted its weapons-grade nuclear plant, South Korean media said.
The Chosun Ilbo newspaper said U.S. spy satellites detected signs of steam at the North's Yongbyon nuclear complex, an indication it may have started reprocessing nuclear fuel. The report, which could not be confirmed, quoted an unidentified government official. South Korea's Yonhap news agency also carried a similar report.
The move would be a major setback for efforts aimed at getting North Korea to disarm.
North Korea had stopped reprocessing fuel rods as part of an international deal. In 2007, it agreed to disable the Yongbyon reactor in exchange for aid and demolished a cooling tower at the complex.
The North has about 8,000 spent fuel rods which, if reprocessed, could allow it to harvest 13 to 18 pounds of plutonium — enough to make at least one nuclear bomb, experts said. North Korea is believed to have enough plutonium for at least a half dozen atomic bombs.
Further ratcheting up tensions, North Korea test-fired five short-range missiles over the past two days, South Korean officials confirmed.
South Korean President Lee Myung-bak urged officials to "remain calm" in the face of North Korean threats, said Lee Dong-kwan, his spokesman.
Pyongyang isn't afraid of any repercussions for its actions, a North Korean newspaper, the Minju Joson, said Wednesday.
"It is a laughable delusion for the United States to think that it can get us to kneel with sanctions," it said in an editorial. "We've been living under U.S. sanctions for decades, but have firmly safeguarded our ideology and system while moving our achievements forward. The U.S. sanctions policy toward North Korea is like striking a rock with a rotten egg."
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