North Korea, facing international censure for this week's nuclear test, threatened today to attack the South after it joined a US-led plan to check vessels suspected of carrying equipment for weapons of mass destruction.
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.
Russia today called in the North Korean ambassador and expressed its "deep regret" over the nuclear test, the Russian foreign ministry said this evening.
In Moscow, news agencies quoted an official as saying that Russia is taking precautionary security measures because it fears tensions over the test could lead to nuclear war.
The UN 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.
North Korea's latest threat came after Seoul announced, following the nuclear test, it was joining the US-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 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 so he can better engineer his succession and divert attention from the country's 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 that he wants his third son to be the next leader of Asia's only communist dynasty.
There may be little the international community can do to deter the North, which has been punished for years by sanctions and is so poor it relies on aid to feed its 23 million people.
South Korean President Lee Myung-bak and Russian President Dmitry Medvedev agreed in a phone call a strong international response was needed, including UN action, Mr Lee's office said.
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 and there were no signs yet that the North, which conducted its only prior nuclear test in October 2006, was again separating plutonium.
North Korea's meagre 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.
Reuters