North Korea Threatens South, Armistice Off

North Korea threatened to attack South Korea one day after Seoul announced that it would join a U.S.-led naval exercise, aimed at intercepting any shipments suspected of carrying materials used in the making of weapons of mass destruction.

North Korean media reported that Pyongyang is feeling "compelled to take a decisive measure" and will mete out "unimaginable and merciless punishment" to any country that will blockade its ships. It warned the international community that it would not hesitate to use its "tremendous military muscle" against any threats.

The government also proclaimed that the North would no longer honor the North-South armistice signed at the end of the Korean War.

VIDEO: North Koreas Military Muscle Play

Also today, South Korean media reported that Pyongyang has restarted a plant that makes plutonium, a material which can be used in the making of nuclear bombs. That report has not been confirmed.

The South Korean decision to join the patrols marked the end of a long debate in Seoul. North Korea has said in the past that such a decision would amount to a "declaration of war."

Today, the North Korean official news agency KCNA reported, an unnamed military official in Pyongyang said, "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."

Today's threats follow a series of short-range missile launches by the North as well as an underground nuclear test.

The flurry of North Korean activity is testing the international community as the U.N. Security Council continues to debate the best way to curb Pyongyang's nuclear ambitions.

North Korea's actions "are a very provocative and destabilizing series of actions," and "they pose a clear threat to international peace and security," Susan Rice, the U.S. ambassador to the United Nations said Tuesday on ABC's "Good Morning America."

And Rice said that North Korea's attempt to provoke the world is failing.

"The message we are sending back is that the international community will not be intimidated and the pressure on North Korea will only increase if they continue on this course," she told "GMA."

Noting that Russia, China, South Korea and Japan have all condemned North Korea's actions, President Obama, who has pledged to more aggressively pursue diplomatic solutions for global challenges, said on Monday that the country is "inviting stronger international pressure." As a result, he added, we will "redouble our efforts toward a more robust international non-proliferation regime that all countries have responsibilities to meet."

Russia Pledges to Work With Seoul to Tackle Pyongyang

But U.S. officials said a push for sanctions against North Korea will depend on the willingness of China and Russia, which hold Security Council vetos.

Today, the Russian news agency Itar-Tass quoted a Foreign Ministry official who said that the "war of nerves" between the two Koreas should not be allowed to develop into a military conflict, after Pyongyang's refusal to stand by the armistice. "A dangerous brinkmanship, a war of nerves, is under way, but it will not grow into a hot war," the official said, adding, "Restraint is needed."

Russian President Dmitry Medvedev told South Korean President Lee Myung-bak today that Moscow would work together with Seoul on a U.N. Security Council resolution. In the past, Russia has been reluctant to support U.S. demands for sanctions.

The Russian news agency Interfax quoted an unnamed security source who said that Moscow is adopting some security measures in its Far Eastern regions, "in case a military conflict, perhaps with the use of nuclear weapons, flares up on the Korean Peninsula."

Although the tests were expected, their timing came as a surprise. U.S. officials said last week that they were monitoring activity at the nuclear test site but said it was difficult to determine whether a test was imminent, given that a lot of that work was underground. Officials also detected activity at the nearby Musudan-Ri, where the long-range Daepodong 2 missile was launched April 5.

Will Pyongyang Return to the Negotiating Table?

Analysts in Seoul say the North Korean leader, Kim Jong Il, is trying to mark his country's presence as a full nuclear power.

"North Korea is making a very clear statement that we are a nuclear state. 'Now deal with us,' as such," said Lee Jung-Hoon, professor of international relations at Yonsei University in Seoul. "It's a wake-up call to the international community that we shouldn't pretend as if somehow North Korea would negotiate dismantlement."

"You can tell that Kim Jong Il seriously feels that there's not much time left. He'll use whatever means he could to increase the bet," said Choi Jinwook, senior researcher at Korea Institute for National Unification.

"North Korea is a bully. If you concede, it will continue to push, and it's pushing and pushing to the point where now, we can't almost go back," said Lee, implying that a harder stance on North Korea is needed to get it to give up the nuclear program. "It's going to be a huge headache, not only for the U.S., but also for the international community as a whole."

ABC News' Jake Tapper, Luis Martinez, Joohee Cho, Kirit Radia and Noriko Namiki contributed to this story, as did The Associated Press and Reuters.