South Korean soldiers using binoculars look at the North Korean side of the border at Dora Observation Post near the village of Panmunjom, South Korea, on Wednesday. (Lee Jin-man/Associated Press)

Amid reports North Korea has restarted a weapons-grade nuclear plant, Pyongyang warned early Wednesday it would launch a military strike against South Korea if it took part in a U.S.-led program to intercept ships suspected of spreading powerful weapons.

The threat comes after North Korea reportedly fired a third short-range missile on Tuesday night off its east coast.

In a statement carried by the North's official Korean Central News Agency, the North's military said the government no longer considers itself bound by the armistice that ended the Korean War.

It warned it would respond with military force against any attempt to stop and search North Korean ships under the Proliferation Security Initiative.

This satellite image, collected May 14, shows the area where North Korea reportedly conducted an underground nuclear test on Monday, about 80 kilometres northwest of the northern city of Kilju. (DigitalGlobe/Associated Press)

"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," a North Korean army spokesman was quoted as saying by the country's official KCNA news agency.

South Korea announced its participation in the anti-proliferation program Tuesday, one day after the North conducted a nuclear test. It said it will join a maritime web of more than 90 nations to intercept ships suspected of spreading nuclear, biological or chemical weapons. North Korea said that the move constitutes an act of war.

The North's military said it cannot guarantee safety for South Korean and U.S. navy ships sailing near the disputed western Korean sea border. The North also accused the United States of hostility on Tuesday and said its army was ready to combat the U.S. in case of an invasion.

U.S. committed to defending South Korea, Japan

Meanwhile, U.S. Secretary of State Hillary Rodham Clinton denounced North Korea's "provocative and belligerent" threats and warned that it must face consequences for its nuclear and missile tests.

She said "North Korea has made a choice" to violate UN Security Council resolutions, ignore international warnings and abrogate commitments made during six-nation nuclear disarmament talks.

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

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

Clinton said she was pleased by 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 implements them.

Despite her tough words, Clinton held out hope that North Korea would return to nuclear disarmament talks and that "we can begin once again to see results from working with the North Koreans toward denuclearization that will benefit, we believe, the people of North Korea, the region and the world."

South Korean media were also reporting that Pyongyang appears to have restarted a plant that makes plutonium, which can be used in nuclear bombs.

According to the reports, satellites have detected signs of steam at the North's Yongbyon nuclear complex, an indication it may have started reprocessing nuclear fuel.

North Korea had stopped reprocessing fuel rods as part of an international deal. But the country is believed to have enough plutonium for at least six atomic bombs.

Yoon Deok-min, a professor at South Korea's state-run Institute of Foreign Affairs and National Security, said North Korea appears still to be in the process of mastering the miniaturization technology required to mount a warhead onto a missile.

Missiles fired

On Tuesday, South Korea's state news agency, Yonhap, reported that ground-to-air and two ground-to-ship missiles with a range of about 130 kilometres were test-fired by the North from an east coast launch pad.

Those launches came a day after North Korea reportedly tested an underground nuclear device believed to be of a size comparable to the bombs dropped on Hiroshima and Nagasaki at the end of the Second World War.

The North is now "more of a threat because they have more data and information about their bomb design," said Daniel Pinkston, a Seoul-based analyst for the International Crisis Group, a Brussels-based think-tank devoted to conflict resolution.

North Korea has been under United Nations sanctions that bar it from nuclear and ballistic activity since its first atomic test in 2006. But Pyongyang had threatened for weeks to carry out its second reported nuclear test after the UN Security Council condemned the April 5 launch of a long-range rocket.

The North claimed the test launch was part of its development of its space program, but other nations alleged it was a test of long-range missile technology.

'Clear violation'

The Security Council condemned the test on Monday as a "clear violation" of a 2006 resolution barring the country from developing its nuclear program.

Pyongyang has been engaged in years of on-off negotiations that have been pressing the impoverished state to give up its nuclear ambitions in return for massive aid and an end to the country's pariah status.

North Korea announced in April that it was withdrawing from the six-nation disarmament talks and said it would restore partly disabled nuclear facilities.

Monday's nuclear test prompted global condemnation with world leaders stating that North Korea was threatening world peace and international security.

U.S. President Barack Obama called the test a "reckless violation of international law."

Obama said the U.S. will defend South Korea and Japan, as world leaders joined together to provide a united response to Pyongyang's nuclear belligerence.

The UN Security Council is still discussing how to approach North Korea's nuclear test.

The five permanent veto-wielding council members — the U.S., Russia, China, Britain and France — along with Japan and South Korea have been in discussions in New York regarding a new UN resolution, which could include sanctions against the North.

Russian Foreign Minister Sergei Lavrov said Wednesday the Security Council should be firm with North Korea but should continue to push to restart international talks with the country.

With files from The Associated Press