PYONGYANG, North Korea (AP) -- Defying U.N. warnings, North Korea on Tuesday conducted an underground nuclear test in the remote, snowy northeast, taking a crucial step toward its goal of building a bomb small enough to be fitted on a missile capable of striking the United States.
North Korea made clear that its third atomic test was a warning to what it considers a hostile United States. Its actions drew immediate condemnation from Washington, the U.N. and others. Even its only major ally, China, voiced opposition.
U.S. President Barack Obama, who was scheduled to give a State of the Union address later Tuesday, 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.
The test was a defiant response to U.N. orders to shut down atomic activity or face more sanctions and international isolation, as well as a direct message from young leader Kim Jong Un to the United States, Pyongyang's No. 1 enemy since the 1950-53 Korean War.
The nuclear test is North Korea's first since Kim 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., North Korea's longtime enemy
North Korea's rocket launches and nuclear tests are largely seen by analysts as threats designed to force the United States to confront the issue of military tensions between the foes.
Pyongyang claimed the device was smaller than in previous tests; Seoul said it likely produced a bigger explosion.
"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.
KCNA said the test is aimed at coping with "the ferocious hostile act of the U.S." -- a reference to what Pyongyang calls Washington's attempts to block its right to send satellites into space.
The timing of the test -- in an underground tunnel -- is significant. It came hours before 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.
Kim Mun Chol, a 42-year-old Pyongyang citizen, called it a strong message to the world.
The test shows the world that North Korea is a "nuclear weapons state that no one can irritate," he told The Associated Press in the North Korean capital. "Now we have nothing to be afraid 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.
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.
If Tuesday's test was indeed successful, as claimed, it 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.
"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 test also shows North Korea is "more confident in their military technology and their military power," he said. "Now they will be emboldened as they focus on other goals."
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.
The test 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.
China expressed firm opposition to the test but called for a calm response by all sides.
North Korea cites what it calls a U.S. military threat in the region as a key reason behind its drive to build nuclear weapons. The two countries fought on opposite sides of the Korean War, which ended after three years with an armistice signed on July 27, 1953, not a peace treaty. The U.S. stations more than 28,000 troops in South Korea to protect the ally.
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.
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."
Associated Press writers Kim Kwang Hyon in Pyongyang, North Korea; Foster Klug, Hyung-jin Kim, Youkyung Lee and Sam Kim in Seoul, South Korea, and Yuri Kageyama and Malcolm Foster in Tokyo contributed to this report.