The communist North's military said in a statement Wednesday that it will respond with "immediate, strong military measures" if the South actually stops and searches any North Korean ships under the Proliferation Security Initiative.
The statement, carried by the North's Korean Central News Agency, said North Korea no longer considers itself bound by the armistice that ended the Korean War, as a protest over the South's participation.
South Korea announced its participation in the anti-proliferation program Tuesday, one day after the North conducted a nuclear test.
"Without fanfare, South Korea signed onto the Proliferation Security Agreement this week and President Obama welcomed the decision - which means South Korea could possibly confront a North Korean vessel," CBS News foreign affairs analyst Pamela Falk said, "and the situation would change rapidly from missile tests to confrontation."
Earlier Wednesday, North Korea restarted its weapons-grade nuclear power plant and fired off its sixth short-range missile, news reports said, deepening a standoff with world powers following its latest nuclear test.
U.S. military surveillance planes flying out of Japan may have witnessed the test launches on Tuesday, but the Pentagon has yet to confirm that missiles were fired, reports CBS News correspondent Barry Petersen.
The missile launches over the past two days came as the U.N. Security Council debated possible new sanctions against the isolated communist nation for its nuclear test on Monday. Retaliatory options were limited, however, and no one was talking publicly about military action.
South Korea's mass-circulation Chosun Ilbo newspaper reported that U.S. spy satellites have detected steam coming from a nuclear facility at North Korea's main Yongbyon plant, indicating the North is reprocessing spent nuclear fuel rods to harvest weapons-grade plutonium.
Its report quoted an unnamed official. South Korea's Defense Ministry and the National Intelligence Service - the country's main spy agency - said they cannot confirm the report.
The North had said it would begin reprocessing in protest over international criticism of its April 5 rocket launch.
North Korea is believed to have enough plutonium for at least half a dozen atomic bombs. The North also has about 8,000 spent fuel rods which, if reprocessed, could allow the country to harvest 13-18 pounds of plutonium - enough to make at least one nuclear bomb, experts said.
North Korea test-fired three short-range missiles Tuesday, including one late at night, from the east coast city of Hamhung, according to South Korea's Yonhap news agency. South Korea's spy chief confirmed two other missiles were launched Monday, but reports put the number at three Monday for a total of six.
Pyongyang also warned ships to stay away from waters off its western coast this week, a sign it may be gearing up for more missile tests, South Korea's coast guard said.
North Korea is "trying to test whether they can intimidate the international community" with its nuclear and missile activity, said Susan Rice, U.S. ambassador to the United Nations.
"But we are united, North Korea is isolated and pressure on North Korea will increase," Rice said.
Diplomats at the U.N. are working on a Security Council resolution that Rice says will levy new sanctions. They might even include enforcing an earlier resolution to stop and search North Korean cargo ships at sea, reports CBS News correspondent David Martin.
"We're going to increase the pressure. We're prepared to step up our efforts to intercept and interdict prohibitive cargo from North Korea," Rice told CBS' The Early Show.
"North Korea has become the world's nuclear pirate by upping the ante with each new round of denuclearization agreements, and there are more divisions among Security Council members with each missile test," said Falk reporting from the U.N. "Although the five permanent members of the Council as well as Japan and South Korea are huddling to draft a new, tougher resolution, China appears to continue to want to work within existing sanctions rather than to impose new ones."
Petersen reports that Beijing is having to tread very carefully in how it deals with its suddenly-more-belligerent ally. North Korea is presently a huge importer of Chinese goods, but that cross-border flow could turn into a flood of North Korean refugees seeking safety in China if tensions escalate too far. (Click here to read more from Petersen.)
"I think the country that has the toughest situation here is actually China, which really has to decide how hard they want to press North Korea," Wendy Sherman, a former advisor to President Clinton, told Martin.
The U.N. Security Council swiftly condemned North Korea's nuclear test on Monday as "a clear violation" of a 2006 resolution banning them and said it will start work immediately on a new one that could result in stronger measures against the reclusive nation.
Even China and Russia - North Korea's closest allies - joined Western powers and representatives from the rest of the world on the council to voice strong opposition to the underground explosion.
After a brief emergency meeting held at Japan's request, the council demanded that North Korea abide by two previous resolutions, which among other things called for Pyongyang to return to six-party talks aimed at eliminating its nuclear program. It also called on all other U.N. member states to abide by sanctions imposed on the North.
Rice told CBS Early Show co-anchor Harry Smith the North Korean regime, "needs to understand that its actions have consequences. The international community, the United States, Russia, China, Japan, South Korea, we're not going to walk away and just throw up our hands and say, 'Let them pursue this path.' They will pay a price for their actions."
Rice said she expected the coming meeting of the Security Council to yield a new resolution on Pyongyang which she believes "will have teeth in it.
"I expect additional sanctions. The pressure will increase on North Korea, economically and otherwise," the ambassador told Smith.
Asked whether the reclusive communist nation's defiance during the previous 48 hours had prompted the Obama administration to more seriously consider a military response, Rice told CBS News the government would "take the steps that are necessary to protect our country and our people.
"We are still focused on what united pressure we can continue to muster and mount to make North Korea recognize that the path it's on is self-destructive and unacceptable," she added.
But many experts believe what's really needed is a high-level envoy like former president Carter, who met with the so-called great leader Kim Il Sung to defuse a crisis in 1994. His son, Kim Jong Il, now rules the hermit kingdom.
"It doesn't make any sense to go unless you're going to talk to Kim Jong Il himself," David Sherman of the Institute for Science and International Security told Martin. "I mean you have to talk to the leader to figure this out and that's, anything else doesn't make any sense at this point."