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.
Adding to tensions in the region, South Korean media reported that Pyongyang had restarted a plant that makes plutonium that can be used in nuclear bombs.
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.
Russia also called the North Korean ambassador to the foreign ministry and told him Moscow has "serious concern" over this week's test, the ministry said.
U.S. Secretary of State Hillary Clinton reaffirmed U.S. commitments to allies Japan and South Korea, said North Korea was behaving in a "provocative and belligerent manner" toward its neighbors, and that there were consequences to such behavior.
Both Moscow and Washington said they hoped North Korea would return to the six-party talks aimed at ending its nuclear program.
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 former U.S. President George W. Bush's administration.
'POWERFUL MILITARY STRIKE'
"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 North Korea no longer was 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.
Diplomats in New York said that the five permanent council members -- the United States, Britain, France, China and Russia -- and Japan agreed at a meeting on Tuesday to beef up and expand existing U.N. sanctions against Pyongyang. South Korea, which is not a council member, was also present.
"There is a clear commitment by the (seven countries) to go for sanctions," a U.N. diplomat close to the talks but who declined to be identified said on Wednesday. "There was no reluctance that I could notice from either China or Russia."
Diplomats said the seven countries would not have a draft sanctions resolution ready to circulate to the full 15-nation council before next week at the earliest.
One diplomat said possible steps include a ban on importing and exporting all arms and not just heavy weapons, asset freezes and travel bans for North Korean officials, placing more firms on a U.N. blacklist and adding goods to it. The diplomats said cargo inspections were also possible, though China is reluctant.
The measures would expand on sanctions approved by the council after Pyongyang's 2006 nuclear test, penalties that have been widely ignored and left unenforced.
CONCERN IN RUSSIA
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.
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.[ID:nSEO269956]. The main index has fallen 3 percent this week. The won currency was also down.
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 an economy that has nearly collapsed 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.
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.
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.
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.
(Additional reporting by Jack Kim, Rhee So-eui and Kim Junghyun in Seoul, Isabel Reynolds in Tokyo and Oleg Shchedrov in Moscow, Arshad Mohammed in Washington, Louis Charbonneau at the United Nations; Editing by Jonathan Thatcher, Bill Tarrant and Will Dunham)