Russia's Defense Ministry confirmed an atomic explosion at 9:54 a.m. (0054 GMT) in northeastern North Korea, estimating the blast's yield at 10 to 20 kilotons -- comparable to the bombs that flattened Hiroshima and Nagasaki.
Hours later, the regime test-fired three short-range, ground-to-air missiles, the Yonhap news agency reported, citing unnamed sources. U.N. Security Council resolutions bar North Korea engaging in any ballistic missile-related activity.
President Barack Obama called the moves "blatant defiance" of the Security Council and a violation of international law that would only further isolate North Korea.
North Korea's claims "are a matter of grave concern to all nations," he said, calling for international action in a statement from Washington. "North Korea's attempts to develop nuclear weapons, as well as its ballistic missile program, constitute a threat to international peace and security."
British Prime Minister Gordon Brown condemned the test as "erroneous, misguided and a danger to the world."
Japanese Prime Minister Taro Aso said the Security Council will meet at 4:30 p.m. Monday in New York (2030 GMT).
"North Korea's nuclear test poses a grave challenge to nuclear nonproliferation and clearly violates U.N. Security Council resolutions," he said in Tokyo. "We are not tolerating this at all."
Even China, North Korea's traditional ally, issued rare criticism of Pyongyang, with the Foreign Ministry saying in a statement posted on its Web site that Beijing was "resolutely opposed" to the test.
North Korea's bold defiance raises the stakes in the standoff over its nuclear program. In the past two months, Pyongyang has launched a rocket despite international calls for restraint; abandoned international nuclear negotiations; restarted its nuclear plants; and warned it would carry out the atomic test as well as long-range missile tests.
The rise in tensions comes amid questions about who will succeed impoverished North Korea's authoritarian leader, 67-year-old Kim Jong Il, who is believed to have suffered a stroke last August. North Korea also has custody of two American journalists -- accused of entering the country illegally and engaging in "hostile acts" -- who are set to stand trial in Pyongyang on June 4.
"This is a political act more than a military act," said Jim Walsh, an international security expert at the Massachusetts Institute of Technology.
Walsh said domestic factors related to North Korea's political transition were likely the main factor.
Monday's atomic test was conducted 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.
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 also angered China and drew wide-ranging sanctions from the Security Council.
An emergency siren sounded in the Chinese border city of Yanji, 130 miles (200 kilometers) to the northwest. A receptionist at Yanji's International Hotel said she and several hotel guests felt the ground tremble.
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" than in 2006.
Pyongyang is believed to have enough weaponized plutonium for at least a half-dozen atomic bombs. However, experts say scientists have not yet mastered the miniaturization needed to mount a nuclear device onto a long-range missile.
Ten to 20 kilotons would be far more than North Korea managed in 2006. U.S. intelligence officials said the 2006 test measured less than a kiloton; 1 kiloton is equal to the force produced by 1,000 tons of TNT. However, Russia estimated the force of the 2006 blast at 5 to 15 kilotons, far higher than other estimates at the time.
Radiation levels in Russia's Primorye region, which shares a short border with North Korea, were normal Monday several hours after the blast, the state meteorological office said.
In Vladivostok, a city of 500,000 people about 85 miles (140 kilometers) from the Russian-North Korean border, translator Alexei Sergeyev said he wasn't concerned about the test and doesn't fear North Korea.
"Their nuclear program does not have military aims -- their only aim is to frighten the U.S. and receive more humanitarian aid as a result," said Sergeyev.
The reported test-firing of short-range missiles took place at the Musudan-ri launchpad on North Korea's northeast coast, some 30 miles (50 kilometers) from the nuclear test site, Yonhap said. Unnamed sources described it as a ground-to-air missile with a range of 80 miles (130 kilometers).
Japan's coast guard said Friday that North Korea warned ships to avoid waters off the coast near the launch site, suggesting Pyongyang was preparing for a missile test. Yonhap also reported brisk activity along the northeast coast last week.
South Korean troops were on high alert but there was no sign North Korean soldiers were massing along the heavily fortified border dividing the two nations, according to an official at the Joint Chiefs of Staff headquarters in Seoul. He spoke on condition of anonymity, citing agency policy.
The two Koreas technically remain at war because their three-year conflict ended in a truce, not a peace treaty, in 1953.
Tensions have been high since conservative President Lee Myung-bak took office in Seoul in February 2008 saying Pyongyang must fulfill its promises to dismantle its nuclear program before it can expect aid.
South Koreans, meanwhile, were grappling with the suicide two days earlier of Lee's liberal predecessor, Roh Moo-hyun, whose death elicited condolences from Kim Jong Il. Kim held a 2007 summit in Pyongyang with Roh, who championed reconciliation with North Korea.
North Korea had agreed in February 2007 to a six-nation pact to begin disabling its main nuclear reactor in exchange for 1 million tons of fuel oil and other concessions. But Pyongyang abruptly halted the process last summer over a dispute with Washington over how to verify its 18,000-page list of past atomic activities.
Talks hosted by Beijing in December failed to resolve the impasse, and North Korea abandoned the six-nation negotiations last month in anger over the U.N. condemnation of its rocket launch.
North Korea claims it launched the rocket to send a satellite into space; South Korea, Japan and other nations saw it as a way to test the technology used to launch an intercontinental ballistic missile, one capable of reaching the U.S.
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