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.

The Committee for the Peaceful Reunification of Korea said in a statement carried by state media that the North is “compelled to take a decisive measure” against South Korea. South Korea’s military said Wednesday it’s prepared to “respond sternly” to any North Korean provocation.

Seoul announced it was going to join The Proliferation Security Initiative (PSI) on Tuesday, after North Korea carried out an underground nuclear test.  The deal drawn up by the US in 2003 is an international effort aimed at preventing the transfer of illegal weapons and weapons technology.

In a statement issued by its official news agency, North Korea said:

“As declared to the world, our revolutionary forces will consider the full participation in the Proliferation Security Initiative by [South Korean President] Lee Myung-bak’s group of traitors as a declaration of war against us.”

South Korean soldiers use binoculars to look at North Korean side as they visit an observation post in Paju near the Demilitarised Zone (DMZ) dividing two Koreas on May 27, 2009 (AFP Photo / Jung Yeon-Je)

“Our military will no longer be bound by the armistice accord as the US has drawn its puppets [South Korea] into the PSI.”

Meanwhile, members of the UN Security Council unanimously denounced North Korea's actions as a great threat and promised a tough response.

“We share a common set of objectives, which are to convey very clearly and unequivocally that the actions by North Korea run counter to the interest of regional peace and security, violate international law, and need to be dealt with directly and seriously,” Susan Rice, US Ambassador to the UN said.

Yukio Takasu, Japan’s Ambassador to the United Nations, said that, despite all the warnings from the Security Council, “The DPRK went ahead with a second nuclear test after a missile launch in April.” He said this is a threat to security, including the security of Japan.

According to reports, North Korea has fired its fifth short-range missile in two days.

"The parties…noted that the North Korean nuclear test is a direct violation of the UN SC Resolution 1718 and contradicts international legal norms," the Kremlin added.

Also, it was said that both countries "intend to actively take part in working out a new UN Security Council resolution on North Korea and continuing close consultations with partners at the Six Party Talks on the Korean peninsula nuclear problem."

Korea restarts nuclear plant

South Korean state news agency Yonhap, citing an unnamed “informed source”, said the North had restarted its nuclear reprocessing facility at Yongbyon in mid-April to produce plutonium.

The nuclear reactor at the facility was to be shut down under a deal reached at Six Party Talks in 2007. However, in response to the UN Security Council’s condemnation of a long-range missile launch on April 5 this year, Pyongyang said it would restart the facility.

The UN Security Council resolution 1718 bans any supplies, sales or transfer of conventional military equipment and armaments to North Korea, and any materials, equipment, products or technologies which could be used by the country in its is programmes to create weapons of mass destruction.

On Wednesday, Yonhap, citing the source, wrote, “the doors of the site where the North stores spent fuel rods were opened ‘several times’ in mid-April, and from late April, plumes of steam were detected at a reprocessing plant.”

“Since early May, we have continuously been seeing steam rising from the nuclear fuel fabrication plant,” the source is quoted as saying.

Russia’s reaction

If N. Korea’s nuclear tests went wrong, Russia would be one of the first to feel the fallout.

The aftershocks from Monday’s blast were felt in the country's Far East, but Moscow is still cautious in its reaction as it wants to make sure any sanctions imposed will not be politically motivated.

“The first immediate political response was quick, but now we need to take some time to reflect on what the specific elements of the future resolution need to be,” Russia’s envoy to the UN Vitaly Churkin said.

Russian Foreign Minister Sergey Lavrov has named two basic principles that define Moscow’s position on a draft UN Security Council resolution.

“First, the Security Council must make a firm statement and work out measures that would effectively prevent further erosion of the non-proliferation regime. Second, in a resolution we must find a way to create conditions for resuming the Six Party Talks. We should not punish the DPRK simply for the sake of punishing it,” Lavrov said.

On Tuesday, following North Korea’s underground test, Moscow indefinitely postponed the meeting of the Russian-North Korean intergovernmental commission on economic and scientific cooperation.

Interfax agency, citing a source at the Russian Foreign Ministry, writes that Moscow believes a new UN Security Council resolution should contain sanctions.

“If the resolution is not under Chapter 7 [of the UN Charter, which deals with threats to peace], then in fact this will be a political declaration, because it will not be implemented,” a source is quoted as saying.

“Given the tough and unanimous reaction which this [nuclear] test aroused around the world and further actions, then of course one should not expect that this would cost just a light reprimand,” he said.

The source went on to say:

“In this case we should give a clear answer – otherwise the Security Council will lose force and respect.”

However, Sergey Mironov, the speaker of the upper chamber of Russia’s Federation Council, talking to journalists on Wednesday, said he believes the international community should stay calm and not hasten sanctions against North Korea.

“Of course, Pyongyang’s nuclear policy can’t be met with enthusiasm,” he said answering to Itar-Tass questions, noting that the tests took place “only 130 kilometers from our borders.”

Also, Itar-Tass quotes a Russian diplomat who said Moscow is concerned about the dangerous development of the situation on the Korean peninsula.

“The situation is anxious and tense. We believe that North Korea plays the brinkmanship game. We face ‘the war of nerves’ that may be turned into a hot war. Restraint is necessary,” the diplomat said.

Another source at the Foreign Ministry told Interfax that Russia doesn’t justify Pyongyang’s actions, but calls on the international community to take North Korean security concerns into consideration.

“North Korea has its rightful concerns, and we admit them. We can’t accept an approach when North Korea is seen as just a wrong country and nothing can be done about it. Whether you like it or not, North Korea is a sovereign state, a UN member and has its legal interests and concerns,” he is quoted.

Russia monitors situation on border with N. Korea

The Russian military have been monitoring the situation in the area bordering North Korea since Monday morning, according to the General Staff of Russia’s Armed Forces, quoted by Itar-Tass news agency.

“The monitoring is carried out by units of various arms and services at the sites of their permanent location,” a General Staff official said.

If the situation changes it is to be reported to the General Staff immediately – especially if it may pose a threat to the Russian Far Eastern region.

However, he noted, no Russian military units have been either reinforced or moved closer to the border.