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Updated 9:22 a.m. ET
PYONGYANG, North Korea North Korea said the atomic test it conducted Tuesday in the remote, snowy northeast was merely its "first response" taken with "maximum restraint," in response to what it called U.S. threats, and said it will continue with unspecified "second and third measures of greater intensity" if Washington maintains its hostility.
The underground test, which set off powerful seismic waves, was a crucial step toward its goal of building a bomb small enough to be fitted on a missile capable of striking the United States. The test drew immediate condemnation from Washington, the U.N. and others. Even its only major ally, China, voiced opposition.
After the launch was first reported, U.S. officials confirmed that there are indications that North Korea intends to conduct a second test of its arsenal, reports CBS News correspondent Margaret Brennan.
A source familiar with the regional threat assessment says that North Korea has been considering doing "two for the price of one," in other words, conduct two tests in anticipation of punishing U.N. action.
CBS News correspondent David Martin reports that the second test could be a missile, according to intelligence assessments made in recent days.
North Korea has been warning of the nuclear test for weeks, but U.S. officials consider the timing of the test to be especially provocative as it comes during the Chinese New Year holiday week, ahead of China's political transition, and on the cusp of the U.S. State of the Union address.
President Barack Obama said nuclear tests "do not make North Korea more secure." Instead, North Korea has "increasingly isolated and impoverished its people through its ill-advised pursuit of weapons of mass destruction," he said in a statement.
North Korea claimed the device was smaller than in previous tests; Seoul said it likely produced a bigger explosion.
The Office of the Director of National Intelligence confirmed the test Tuesday, saying in a statement: "The U.S. Intelligence Community assesses that North Korea probably conducted an underground nuclear explosion in the vicinity of P'unggye on February 12, 2013. The explosion yield was approximately several kilotons. Analysis of the event continues."
The test was a defiant response to U.N. orders to shut down atomic activity or face more sanctions and international isolation. It will likely draw more sanctions from the United States and other countries at a time when North Korea is trying to rebuild its moribund economy and expand its engagement with the outside world.
The Security Council will meet on Tuesday to discuss its reaction to the test, Reuters reports.
CBS News foreign affairs analyst Pamela Falk reports that China's backing of new sanctions after the North's December rocket launch is a strong indication that the Security Council will be able to pass a tough new resolution. There is new leadership in China, South Korea, North Korea and Japan and there is an increasing concern in the international community that Pyongyang's nuclear test could destabilize the region.
Several U.N. resolutions bar North Korea from conducting nuclear or missile tests because the U.N. Security Council considers Pyongyang a would-be proliferator of weapons of mass destruction and its nuclear testing a threat to international peace and stability. North Korea dismisses that as a double standard, and claims the right to build nuclear weapons as a defense against the United States, which has been seen as enemy No. 1 since the 1950-53 Korean War. The U.S. stations more than 28,000 troops in South Korea to protect its ally.
North Korea appears defiant in the face of further sanctions, telling the U.N. disarmament forum on Tuesday that it would never bow to resolutions on its nuclear program and that prospects were "gloomy" for the denuclearisation of the divided Korean peninsula because of a "hostile" U.S. policy, Reuters reports.
"The U.S. and their followers are sadly mistaken if they miscalculate the DPRK would respect the entirely unreasonable resolutions against it. The DPRK will never bow to any resolutions," Jon Yong Ryong, first secretary of North Korea's mission in Geneva, told the Conference on Disarmament, referring to the Democratic People's Republic of Korea (DPRK).
He said: "If the EU (European Union) truly wants peace and security on the Korean peninsula, it should urge the U.S. first to terminate its hostile policy towards DPRK on an impartial basis."
Tuesday's test is North Korea's first since young leader Kim Jong Un took power of a country long estranged from the West. The test will likely be portrayed in North Korea as a strong move to defend the nation against foreign aggression, particularly from the U.S.
"The test was conducted in a safe and perfect way on a high level, with the use of a smaller and light A-bomb, unlike the previous ones, yet with great explosive power," North Korea's official Korean Central News Agency said, confirming speculation that seismic activity near Kilju around midday was a nuclear test.
North Korea was punished by more U.N. sanctions after a December launch of a rocket that the U.N. and Washington called a cover for a banned missile test. Pyongyang said it was a peaceful, and successful, bid to send a satellite into space.
The timing of the test is significant. It came hours before Mr. Obama's speech and only days before the Saturday birthday of Kim Jong Un's father, late leader Kim Jong Il, whose memory North Korean propaganda has repeatedly linked to the country's nuclear ambitions.
This year also marks the 60th anniversary of the signing of the armistice that ended the 1950-53 Korean War, and in late February South Korean President-elect Park Geun-hye will be inaugurated.
In Pyongyang, where it was snowing Tuesday, North Koreans gathered around televisions to watch a 3 p.m. TV broadcast announcing the nuclear test.
The test shows the world that North Korea is a "nuclear weapons state that no one can irritate," Kim Mun Chol, a 42-year-old Pyongyang citizen, told The Associated Press in the North Korean capital. "Now we have nothing to be afraid of in the world."
The National Intelligence Service in Seoul told lawmakers that North Korea may conduct an additional nuclear test and test-launch a ballistic missile in response to U.N. talks about imposing more sanctions, according to the office of South Korean lawmaker Jung Chung-rae, who attended the private meeting. Analysts have also previously speculated that Pyongyang might conduct multiple tests, possibly of plutonium and uranium devices.
North Korea is estimated to have enough weaponized plutonium for four to eight bombs, according to American nuclear scientist Siegfried Hecker.
It wasn't immediately clear to outside experts whether the device exploded Tuesday was small enough to fit on a missile, and whether it was fueled by plutonium or highly enriched uranium. A successful test would take North Korean scientists a step closer to building a nuclear warhead that can reach U.S. shores -seen as the ultimate goal of North Korea's nuclear program.
In 2006, and 2009, North Korea is believed to have tested devices made of plutonium. But in 2010, Pyongyang revealed a program to enrich uranium, which would give the country a second source of bomb-making materials - a worrying development for the U.S. and its allies.
"This latest test and any further nuclear testing could provide North Korean scientists with additional information for nuclear warhead designs small enough to fit on top of its ballistic missiles," Daryl Kimball and Greg Thielmann wrote on the private Arms Control Association's blog. "However, it is likely that additional testing would be needed for North Korea to field either a plutonium or enriched uranium weapon."
Uranium would be a worry because plutonium facilities are large and produce detectable radiation, making it easier for outsiders to find and monitor. However, uranium centrifuges can be hidden from satellites, drones and nuclear inspectors in caves, tunnels and other hard-to-reach places. Highly enriched uranium also is easier than plutonium to engineer into a weapon.
Monitoring stations in South Korea detected an earthquake in the North with a magnitude of 4.9 and the South's Defense Ministry said that corresponds to an estimated explosive yield of 6-7 kilotons.
The yields of the North's 2006 and 2009 tests were estimated at 1 kiloton and 2 to 6 kilotons, respectively, spokesman Kim Min-seok said. By comparison, U.S. nuclear bombs that flattened Nagasaki and Hiroshima during World War II were estimated at 13 kilotons and 22 kilotons, respectively, Kim said.
The test is a product of North Korea's military-first, or songun, policy, and shows Kim Jong Un is running the country much as his father did, said Daniel Pinkston of the International Crisis Group think tank.
The decision to push ahead with a test will be a challenge to the U.N. Security Council, which recently punished Pyongyang for launching the December long-range rocket. In condemning that launch and imposing more sanctions on Pyongyang, the council had demanded a stop to future launches and ordered North Korea to respect a ban on nuclear activity - or face "significant action" by the U.N.
U.N. chief Ban Ki-moon condemned the test in a statement. Japanese officials said they expected the Security Council to meet later to take up the nuclear test.
China expressed firm opposition to the test but called for a calm response by all sides.
The other part of a credible North Korean nuclear deterrent is its missile program. While it has capable short and medium-range missiles, it has struggled in tests of technology for long-range missiles needed to carry bombs to the United States, although it did launch the satellite in December.
North Korea isn't close to having a nuclear bomb it can use on the United States or its allies. Instead, Hecker said in a posting on Stanford University's website, "it wants to hold U.S. interests at risk of a nuclear attack to deter us from regime change and to create international leverage and diplomatic maneuvering room."