HONG KONG (CNN) -- North Korea's nuclear test prompted united condemnation and a vow of tough action from the U.N. Security Council on Tuesday.
The council issued a statement slamming an underground nuclear test that jolted the already fragile security situation in Northeast Asia.
South Korean Foreign Minister Kim Sung-hwan, repeating a statement to reporters outside council chambers, said the test violated council resolutions, and "there continues to exist a clear threat to international peace and security."
Kim said that council members will start work "on appropriate measures in a Security Council resolution" and that "North Korea will be held responsible for any consequences of this provocative act." The council is chaired this month by South Korea.
The statement comes after the Security Council last month promised "significant action" if another North Korean nuclear test occurred.
The test probably took place near P'unggye and yielded "several kilotons," according to assessments cited by the U.S. director of national intelligence. It drew condemnation from around the globe and prompted an emergency Security Council session Tuesday morning.
It is the first nuclear test carried out under the North's young leader, Kim Jong Un, who appears to be sticking closely to his father's policy of building up the isolated state's military deterrent to keep its foes at bay, shrugging off the resulting international condemnation and sanctions.
After Kim Sung-hwan spoke, Susan Rice, the U.S. ambassador to the United Nations, told reporters outside the council chambers that the panel met to discuss what she called a "provocative test" and an action that council members regard as "regrettable."
The test violates North Korea's obligations under several unanimous council resolutions, she said.
The council "must and will deliver a swift, credible and strong response" in a resolution to deal with Pyongyang's missile and weapons programs, she said. The goal, she said, is to tighten and augment measures, such as sanctions, against the country.
North Korea "does not and will not benefit from violating international law," said Rice, who stressed that the country has "isolated and impoverished its people from its ill-advised pursuit" of weapons of mass destruction and weapons delivery systems.
The council's actions make it clear that such actions are "not acceptable" and "will not be tolerated," she said.
The permanent members of the United Nations Security Council are the United States, Britain, France, Russia and China.
Although Pyongyang had announced plans for the test in recent, vitriolic statements, its decision to go ahead with it provided a stark reminder of a seemingly intractable foreign policy challenge for President Barack Obama ahead of his State of the Union address later Tuesday.
The test was designed "to defend the country's security and sovereignty in the face of the ferocious hostile act of the U.S.," the North's state-run Korean Central News Agency said, referring to new U.S.-led sanctions on Pyongyang after the recent launch of a long-range rocket.
"This nuclear test is our first measure, which displayed our maximum restraint," KCNA said. "If the U.S. continues with their hostility and complicates the situation, it would be inevitable to continuously conduct a stronger second or third measure."
Tuesday's nuclear test, which follows previous detonations by the North in 2006 and 2009, had greater explosive force and involved the use of a smaller, lighter device, KCNA reported.
North Korea's nuclear program is shrouded in secrecy, so it's almost impossible to independently verify many of the details of the test. But its claims play into fears among the United States and its allies that Pyongyang is moving closer to the kind of miniaturized nuclear device that it can mount on a long-range missile.
The United States will try to determine if North Korea has tested a uranium weapon for the first time, a senior White House official said. The first two were plutonium bombs.
Despite the North's claims of progress Tuesday, analysts say they believe it is still years away from having the technology to deliver a nuclear warhead on a missile.
"This test isn't going to do that in and of itself, but it is a significant step forward," said Mike Chinoy, a senior fellow at the University of Southern California's U.S.-China Institute.
Condemnation across the globe
After Pyongyang confirmed it had gone ahead with the test in defiance of international pressure, world leaders responded with condemnation.
Among those countries criticizing North Korea were the United States, China, Russia, Japan and South Korea -- the five nations that had been in talks with North Korea for years over its nuclear program.
"This is a highly provocative act" that threatens regional stability, breaches U.N. resolutions and increases the risk of proliferation, Obama said in a statement.
"North Korea's nuclear weapons and ballistic missile programs constitute a threat to U.S. national security and to international peace and security," he said, calling for "further swift and credible action by the international community."
South Korea said the test presented "an unforgivable threat to the Korean peninsula's peace and safety."
"North Korea should be responsible for all the serious consequences brought by such an action," said Chun Young-woo, national security adviser to South Korean President Lee Myung-bak, who is near the end of his term in office.
In a statement, Russia's Foreign Ministry demanded that North Korea refrain from any nuclear missile program and adhere to U.N. Security Council guidelines.
It condemned the test as an affront to the community of nations: "It's doubly sad that we are talking about the state with which our country has a long history of good neighborliness."
Japanese Prime Minister Shinzo Abe called the testing a "grave threat to the safety of Japan and a serious challenge against international disarmament framework based on the non-nuclear proliferation treaty."
Military jets in Japan were monitoring radioactive fallout, but all appears to be fine, authorities said.
U.N. Secretary-General Ban Ki-moon called the test "a clear and grave violation of the relevant Security Council resolutions."
The China question
Perhaps the most closely watched reaction came from China, North Korea's main ally and the source of crucial economic and political support to the regime in Pyongyang.
The Chinese Foreign Ministry said it "resolutely opposes" the North's latest test, which it noted had taken place "despite the international community's widespread opposition."
Beijing summoned the North Korean ambassador to China over its "dissatisfaction" with the test, the ministry said.
It said it "strongly" urged North Korean officials to "abide by their promise to denuclearize and take no further action that will worsen the situation."
The real question, though, is whether Beijing will support significantly tougher measures against its smaller neighbor following the test, something it has refrained from doing in the past.
"The Chinese don't like the idea of international sanctions and coercing other countries," Chinoy said. "They still have a strategic interest in maintaining a viable separate North Korea as a buffer against a pro-U.S. South Korea, and that has only become more important as tensions between the U.S. and China have increased."
Recent opinion articles published in the state-run Chinese newspaper Global Times suggested Beijing's patience with North Korea may be wearing thin and raised the prospect of reducing support to Pyongyang.
But with fears in Beijing of what a possible collapse of the North Korean regime could bring, strong measures appear unlikely for the time being.
"I think the key with China right now is that they are necessary to a solution, but we can't expect them to solve the problem for us," said Philip Yun, executive director of the Ploughshares Fund, a U.S.-based foundation that seeks to stop the spread of nuclear weapons.
Indications that the test had taken place first emerged when U.S. seismologists reported a disturbance Tuesday morning in North Korea centered near the site of the secretive regime's two previous atomic blasts.
The area around the epicenter of the tremor in northeastern North Korea has little or no history of earthquakes or natural seismic hazards, according to U.S. Geological Survey maps.
The disturbance reported Tuesday had a magnitude of 5.1 -- upgraded from an initial estimate of 4.9 -- and took place at a depth of about one kilometer, the USGS said.
Kim Min-seok, a spokesman for the South Korean Defense Ministry, said the magnitude of the "artificial tremor" suggested the size of the blast could be in the order of 6 to 7 kilotons, more powerful than the North's two prior nuclear tests.
That calculation, though, was based on the USGS's initial estimate of a 4.9-magnitude seismic disturbance, he said. A 5.1-magnitude tremor could indicate a 10-kiloton explosion.
News breaks amid key dates in Northeast Asia
The test took place at a time when several East Asian countries, including China, North Korea's major ally, are observing public holidays for the Lunar New Year, which began Sunday.
It also comes ahead of significant dates in both North and South Korea.
On Saturday, North Koreans will celebrate the birthday of Kim Jong Il, the former North Korean leader who died in December 2011 after 17 years in power and was succeeded by his son, Kim Jong Un.
And on February 25, the South Korea president-elect, Park Geun-hye, will take office. She had campaigned on a pledge to seek increased dialogue with the North, but Pyongyang's recent moves have left her little room for maneuver.
In a statement Tuesday, Park condemned the nuclear test, saying it harmed ties between the two Koreas.
North Korea announced last month that it was planning a new nuclear test and more long-range rocket launches, all of which it said were part of a new phase of confrontation with the United States.
It made the threats two days after the United Nations Security Council had approved the broadening of sanctions on the reclusive, Stalinist regime in response to the North's launching of a long-range rocket in December that apparently succeeded in putting a satellite in orbit.
Pyongyang said it carried out the launch for peaceful purposes, but it was widely considered to be a test of ballistic missile technology.
Threats against the U.S.?
The North's recent propaganda has used words and images that imply a threat to the United States, but analysts dismiss the prospect that Pyongyang is willing or able to carry out a military attack on U.S. soil.
The latest nuclear test is worrying in military terms, Chinoy said, "but does this mean they can drop a nuclear weapon on Los Angeles? Absolutely not. The notion that they are going to target the U.S. is way off the mark."
U.S. analysts say North Korea's first bomb test, in October 2006, produced an explosive yield at less than 1 kiloton (1,000 tons) of TNT. A second test in May 2009 is believed to have been about 2 kilotons, National Intelligence Director James Clapper told a Senate committee in 2012.
By comparison, the bomb the United States dropped on Hiroshima in 1945 was a 15-kiloton device.
The North's latest test on Tuesday suggests the country has made a notable step forward in terms of power, said Jeffrey Lewis, East Asia director at the James Martin Center for Nonproliferation Studies, part of the Monterey Institute of International Studies.
"They were pretty clear they were going to up the yield a lot," Lewis said, and it "looks like they've done that."
He also warned that Kim Jong Un's regime may not be ready to relinquish the headlines yet, suggesting that a second test remained a possibility and could happen within a few days.
In a commentary last week, the North's KCNA said that Pyongyang had "drawn a final conclusion that it will have to take a measure stronger than a nuclear test to cope with the hostile forces' nuclear war moves."
It didn't elaborate on what would be stronger than a nuclear test.