SEOUL - NORTH Korea threatened military action on Wednesday against US 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.

Pyongyang, reacting angrily to Seoul's decision to join an international program to intercept ships suspected of aiding nuclear proliferation, called the move 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 honour the treaty.

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

North Korea's latest belligerence comes as the UN Security Council debates how to punish the regime for testing a nuclear bomb on 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 US-led network of nations seeking to stop ships from transporting the materials used in nuclear bombs.

North Korea warned on Wednesday that any attempt to stop, board or inspect its ships would constitute a 'grave violation.' -- AP