SEOUL, South Korea - North Korea threatened military action Wednesday against U.S. and South Korean warships plying the waters near the Koreas' disputed maritime border, raising the specter of a naval clash just days after the regime's underground nuclear test.

In Washington, Secretary of State Hillary Rodham Clinton warned that Pyongyang faced unspecified consequences because of its �provocative and belligerent� acts.

Pyongyang, reacting angrily to Seoul's decision to join an international program to intercept ships suspected of aiding nuclear proliferation, called South Korea's decision tantamount to a declaration of war.

�Now that the South Korean puppets were so ridiculous as to join in the said racket and dare declare a war against compatriots,� North Korea is �compelled to take a decisive measure,� the Committee for the Peaceful Reunification of Korea said in a statement carried by state media.

The North Korean army called it a violation of the armistice the two Koreas signed in 1953 to end their three-year war, and said it would no longer honor the treaty.

South Korea's military said Wednesday it was prepared to �respond sternly� to any North Korean provocation.

Clinton said �there are consequences to such actions,� referring to discussions in the United Nations meant to punish North Korea for its nuclear and missile tests.

She also underscored the firmness of the U.S. treaty commitment to defend South Korea and Japan, U.S. allies in easy reach of North Korean missiles.

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.

Ambassadors from the five permanent veto-wielding council members � the United States, Russia, China, Britain and France � as well as Japan and South Korea were working out the details of a new resolution.

The success of any new sanctions would depend on how aggressively China, one of North Korea's only allies, implements them.

�It's not going too far to say that China holds the keys on sanctions,� said Kim Sung-han, an international relations professor at Seoul's Korea University.

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.�

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,� the army said in a statement carried by state media.

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 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.

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 (six to eight kilograms) 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.

Russia's foreign minister said world powers must be firm with North Korea but 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 Security Council resolution that will help restart stalled six-nation talks over North Korea's nuclear programs and will not provoke Pyongyang into even more aggressive activity.

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.�