SEOUL - North Korea, facing international censure for this week's nuclear test, threatened on Wednesday to attack the South after it joined a U.S.-led plan to check vessels suspected of carrying equipment for weapons of mass destruction.

In Moscow, news agencies quoted an official as saying that Russia is taking precautionary security measures because it fears mounting tensions over the test could escalate to war.

Adding to mounting tension in the region, South Korean media reported that Pyongyang had restarted a plant that makes plutonium that can be used in nuclear bombs.

North Korea's latest threat came after Seoul announced, following the North's nuclear test on Monday, it was joining the U.S.-led Proliferation Security Initiative, launched under the George W. Bush administration as a part of its war on terror.

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 official KCNA news agency.

He reiterated that the North was no longer bound by an armistice signed at the end of the 1950-53 Korean War because Washington had ignored its responsibility as a signatory by drawing Seoul into the anti-proliferation effort.

The U.N. Security Council is discussing ways to punish Pyongyang for Monday's test, widely denounced as a major threat to regional stability and which brings the reclusive North closer to having a reliable nuclear bomb.

Interfax news agency quoted an unnamed security source as saying a stand-off triggered by Pyongyang's nuclear test on Monday could affect the security of Russia's far eastern regions, which border North Korea.

We are not talking about stepping up military efforts but rather about measures in case a military conflict, perhaps with the use of nuclear weapons, flares up on the Korean Peninsula, the source said.

Russian President Dmitry Medvedev told South Korean President Lee Myung-bak, who called him on Wednesday, that Russia would work with Seoul on a new U.N. Security Council resolution and to revive international talks on the North Korean nuclear issue.

INVESTOR RISK

Seoul shares closed lower with traders saying the latest rumblings underscored the risks for investors stemming from troubles along the Cold War's last frontier. The main index has fallen 3 percent this week. The won currency was also down.

The nuclear test has raised concern about Pyongyang spreading weapons to other countries or groups. Washington has accused it of trying try to sell nuclear know-how to Syria and others.

The rival Koreas fought two deadly naval clashes in 1999 and 2002 near a disputed maritime border off their west coast and the North has threatened in the past year to strike South Korean vessels in those Yellow Sea waters.

Analysts say Pyongyang's military grandstanding is partly aimed at tightening leader Kim Jong-il's grip on power to better engineer his succession and divert attention from a weak economy, which has fallen into near ruin since he took over in 1994.

Many speculate Kim's suspected stroke in August raised concerns about succession and he wants his third son to be the next leader of Asia's only communist dynasty.

North Korea has been punished for years by sanctions and is so poor it relies on aid to feed its 23 million people, but that has not deterred it from provocations.

A U.S. Treasury Department official said it was weighing possible action to isolate the North financially. A 2005 U.S. clampdown on a Macau bank suspected of laundering money for Pyongyang effectively cut the country off from the international banking system.

The secretive North appears to have made good on a threat issued in April of restarting a facility at its Yongbyon nuclear plant that extracts plutonium, South Korea's largest newspaper, Chosun Ilbo, reported.

There are various indications that reprocessing facilities in Yongbyon resumed operation (and) have been detected by U.S. surveillance satellite, and these include steam coming out of the facility, it quoted an unnamed government source as saying.

The Soviet-era Yongbyon plant was being taken apart under a six-country disarmament-for-aid deal. The surveillance had yet to detect any signs that the North, which conducted its only prior nuclear test in October 2006, was again separating plutonium.

'GRAND UNDERTAKING'

North Korea's meager supply of fissile material is likely down to enough for five to seven bombs after Monday's test, experts have said. It could probably extract enough plutonium from spent rods cooling at the plant for another bomb's worth of plutonium by the end of this year.

Japan's upper house of parliament denounced the test and said in a resolution the government should step up its sanctions.

North Koreans celebrated, with a rally in the capital of top cadres and military brass, KCNA said.

The nuclear test was a grand undertaking to protect the supreme interests of the DPRK (North Korea) and defend the dignity and sovereignty of the country and nation, it quoted a communist party official as saying.

The North's next step may to be resume operations at all of Yongbyon, with experts saying it could take the North up to a year to reverse disablement steps. Once running, it can produce enough plutonium to make one bomb a year.

The hermit state has also threatened to launch a long-range ballistic missile if the Security Council does not apologize for tightening sanctions to punish it for an April launch widely seen as a missile test that violated U.N. measures.

Russia South Korea Dmitry Medvedev