SEOUL, South Korea — The United Nations swiftly condemned North Korea for its test of a powerful nuclear bomb, and South Korean announced Tuesday it would join a U.S.-led initiative to intercept ships suspected of spreading weapons of mass destruction.
The U.N. Security Council said the test was a "clear violation" of a 2006 resolution banning North Korea from conducting nuclear development, and that it would start work immediately on a new resolution that could result in even stronger measures.
Russian officials said the nuclear bomb that the North detonated underground Monday was comparable to those that obliterated Hiroshima and Nagasaki, raising fears that the communist country could spread such technology abroad.
In a further sign of the North's mounting standoff with the world, a report said the country was likely preparing to fire short-range missiles Tuesday off its western coast.
South Korea's Yonhap news agency, citing a defense source it did not identify, said North Korea banned ships from waters off its western coast and would probably fire short-range missiles as early as Tuesday.
A Defense Ministry spokesman in Seoul said he was aware of the report though could not confirm it. He added that the North has routinely issued such shipping bans at this time of year due to military exercises.
South Korean spy chief Won Sei-hoon told lawmakers Tuesday that North Korea fired a ground-to-ship missile from its eastern coast Monday and there is a possibility of another missile launch, according to the office of opposition lawmaker Park Young-sun, who attended the closed-door session.
President Barack Obama told South Korean President Lee Myung-bak that the United States will protect his country from any possible North Korean aggression, Lee's spokesman Lee Dong-kwan said after the two leaders spoke by telephone Tuesday.
Obama and Lee "agreed that the test was a reckless violation of international law that compels action in response," the White House said in a statement on the talks. They also vowed to "seek and support a strong United Nations Security Council resolution with concrete measures to curtail North Korea's nuclear and missile activities."
Obama also spoke by telephone with Japanese Prime Minister Taro Aso, the White House said, with the leaders agreeing to step up coordination with South Korea, China and Russia. Obama also reiterated the U.S. commitment to defend Japan, it said.
South Korea, which previously stayed out of the U.S.-led Proliferation Security Initiative in order to pursue reconciliation efforts with North Korea, set aside its reservations and announced it would join the pact immediately. The program involves stopping and searching ships suspected of carrying nuclear, chemical and biological weapons, materials to make them, or missiles to deliver them.
North Korea previously has warned the South that its joining the program would be considered an act of war.
Earlier, Obama had criticized Pyongyang's "blatant defiance" of existing resolutions. British Prime Minister Gordon Brown condemned the test as a "danger to the world." Russia's Foreign Ministry called it "a serious blow to international efforts" to prevent the spread of nuclear weapons.
On Tuesday, North Korea accused the U.S. of hostility and said its army and people are ready to defeat an American invasion, accusing Obama of attempting to "militarily stifle" the communist country.
"The current U.S. administration is following in the footsteps of the previous Bush administration's reckless policy of militarily stifling North Korea," the North's main Rodong Sinmun newspaper said in a commentary carried by the country's official Korean Central News Agency.
French officials said they would push for new sanctions, and even traditional Pyongyang ally China said it was "resolutely opposed" to the test, which Russian officials estimated yielded a powerful 10- to 20-kiloton blast _ enough to flatten a city and far more than North Korea managed in a 2006 atomic test.
Pyongyang's unprecedented defiance has raised the stakes in the mounting standoff over its nuclear program.
Last month, Pyongyang launched a rocket despite international calls for restraint, abandoned international nuclear negotiations, restarted its nuclear plants and warned it would carry out the atomic and long-range missile tests.
"We're heading for a full-blown crisis with the North," said Peter Beck, a Korean affairs expert who teaches at American University in Washington.
The rise in tensions comes amid speculation about who will succeed North Korea's authoritarian leader, 67-year-old Kim Jong Il, who is believed to have suffered a stroke last August.
Kim, who inherited the leadership from his father in 1994 and rules the nation of 24 million with an iron fist, has three sons but has not publicly named a successor.
Though desperately poor, North Korea increasingly has turned inward. With last month's controversial rocket launch and Monday's nuclear test, Kim clearly wants to show that the nation remains strong, analysts said.
"Kim Jong Il is trying to demonstrate his virility and that they are a power to be reckoned with," Beck said.
Monday's atomic test was conducted shortly before 10 a.m. about 50 miles (80 kilometers) northwest of the northern city of Kilju, Russian Defense Ministry spokesman Alexander Drobyshevsky said, speaking on state-run Rossiya television.
The ministry said it estimated the test's yield at 10 to 20 kilotons.
Kilju, in the northeastern province of North Hamgyong, is where North Korea conducted its first nuclear test in October 2006 in a surprise move that drew wide-ranging sanctions from the Security Council.
North Korea boasted that Monday's test was conducted "on a new higher level in terms of its explosive power and technology of its control."
U.S. and French officials have said the 2006 test measured less than a kiloton; 1 kiloton is equal to the force produced by 1,000 tons of TNT. Russia estimated the force of the 2006 blast at 5 to 15 kilotons, far higher than other estimates at the time.
Associated Press writers Jean H. Lee, Kelly Olsen, Jae-soon Chang, Kwang-Tae Kim and Jae Hee in Seoul, Steve Gutterman in Moscow, John Heilprin in Copenhagen, Edith M. Lederer at the United Nations and Shino Yuasa in Tokyo contributed to this report.