SEOUL, South Korea – North Korea launched a tirade Wednesday against world powers threatening to punish it for conducting its second nuclear test, saying it is not afraid of sanctions and calling South Korea's decision to join an operation to prevent the spread of weapons a declaration of war.
The North also has reportedly restarted its weapons-grade nuclear plant. It staged a rally in its capital, Pyongyang, on Tuesday to celebrate the test.
The isolated communist regime said through its official news agency that it would respond with military action if South Korea tries to stop or search any of its ships as part of the U.S.-led Proliferation Security Initiative.
"Those who provoke (North Korea) once will not be able to escape its unimaginable and merciless punishment," the North's official news agency said.
South Korea decided to join the anti-proliferation initiative on Tuesday.
Meanwhile, South Korea's mass-circulation Chosun Ilbo newspaper reported Wednesday that U.S. spy satellites detected signs of steam at the North's Yongbyon nuclear complex, an indication that it may have started reprocessing nuclear fuel.
The report, which could not be confirmed, quoted an unidentified government official. South Korea's Yonhap news agency also had a similar report.
The move would be a major setback for efforts aimed at getting North Korea to disarm.
North Korea had stopped reprocessing fuel rods as part of an international deal. In 2007, it agreed to disable the Yongbyon reactor in exchange for aid and demolished a cooling tower at the complex.
The North has about 8,000 spent fuel rods which, if reprocessed, could allow it to harvest 13 to 18 pounds (six to eight kilograms) of plutonium — enough to make at least one nuclear bomb, experts said. North Korea is believed to have enough plutonium for at least a half dozen atomic bombs.
Further ratcheting up tensions, North Korea has test-fired five short-range missiles over the past two days, South Korean officials confirmed.
North Korea suggested that more missile tests could be planned, telling ships to stay away from waters off its west coast through Wednesday, according to South Korea's coast guard.
The North's moves have brought a wave of international reproach.
In New York, the U.N. Security Council is discussing a resolution that could include new sanctions.
Ambassadors from the five permanent veto-wielding council members — the United States, Russia, China, Britain and France — as well as Japan and South Korea are expected to meet again soon to work out the details.
Council members, after condemning the test on Monday, said they would follow up with a new legally binding resolution.
China and Russia, both allies of North Korea, slammed it for going ahead with the blast.
Moscow's U.N. ambassador, Vitaly Churkin, who is also the Security Council president, said the 15-member body would begin work quickly on a new resolution, according to ITAR-Tass.
He declined to say if additional sanctions were discussed.
China also said it "resolutely opposed" the nuclear test. But it was unclear how far it would go in sanctioning its neighbor.
Experts said past sanctions have had poor results because they weren't fully implemented and because North Korea is already one of the most isolated countries in the world.
Kim Sung-han, an international relations professor at Seoul's Korea University, said U.N. resolution 1718, adopted after the North's first nuclear test in 2006, has strong elements but has not been strictly observed.
He said the success of any sanctions would depend on how aggressively China implements them.
"It's not going too far to say that China holds all keys in sanctions," he said.
Details of Monday's nuclear test may take days to confirm.
Russian defense officials said the blast was roughly as strong as the bombs dropped on the Japanese cities of Hiroshima and Nagasaki in World War II and was stronger than North Korea's first test in 2006. Other experts said that was probably an overestimate, and put the blast closer in strength to the first test.
North Korea seemed unconcerned by the condemnation.
A large crowd of Pyongyang residents, including senior military and party officials, gathered Tuesday in a stadium to celebrate the nuclear test.
Choe Thae Bok, a high-ranking party official, was quoted by North Korea's official news agency as saying that the nuclear test "was a grand undertaking" to protect the country against "the U.S. imperialists' unabated threat to mount a pre-emptive nuclear attack and (put) sanctions and pressure upon it."
A North Korean newspaper, Minju Joson, said in a commentary Wednesday that Pyongyang does not fear repercussions.
"It is a laughable delusion for the United States to think that it can get us to kneel with sanctions," it said. "We've been living under U.S. sanctions for decades, but have firmly safeguarded our ideology and system while moving our achievements forward. The U.S. sanctions policy toward North Korea is like striking a rock with a rotten egg."