South Korean army soldiers patrol by the national flags and ribbons, wishing for the reunification of the two Koreas, attached on the barbed-wire fence at the Imjingak Pavilion near the border village of Panmunjom, which has separated the two Koreas since the Korean War, in Paju, north of Seoul, South Korea, Wednesday, Feb. 13, 2013. Defying U.N. warnings, North Korea on Tuesday conducted its third 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. (AP Photo/Lee Jin-man)

Map locates an underground nuclear test near the Punggye-ri test facility;

SEOUL, South Korea (AP) — North Korea's neighbors bolstered their military preparations and mobilized scientists Wednesday to determine whether Pyongyang's third nuclear test, conducted in defiance of U.N. warnings, was as successful as the North claimed.

The detonation was also the focus of global diplomatic maneuvers, with U.S. Secretary of State John Kerry reaching out to counterparts in Seoul, Beijing and Tokyo. President Barack Obamaused his State of the Union address to assure U.S. allies in the region and leveled a warning of "firm action."

"Provocations of the sort we saw last night will only isolate them further as we stand by our allies, strengthen our own missile defense and lead the world in taking firm action in response to these threats," Obama said.

The nuclear device detonated Tuesday at a remote underground site in the northeast is seen as a crucial step toward North Korea's goal of building a bomb small enough to be fitted on a missile capable of striking the United States.

North Korea said it tested a "smaller and light A-bomb, unlike the previous ones, yet with great explosive power." Still, just what happened in the test is unclear to outsiders.

Intelligence officials and analysts in Seoul raised the possibility of another nuclear test and ballistic missile test-launches, and North Korea said its latest test was merely its "first response" to what it called U.S. threats. The North's Foreign Ministry said Pyongyang will continue with unspecified "second and third measures of greater intensity" if Washington maintains its hostility.

South Korea has raised its military readiness alert level, and on Wednesday Seoul used aircraft and ships, as well specialists on the ground, to collect air samples to analyze possibly increased radiation from the test, according to Seoul's Defense Ministry. Japanese fighter jets were dispatched immediately after the test to collect atmospheric samples. Japan has also established monitoring posts, including one on its northwest coast, to collect similar data.

Underground nuclear tests often release radioactive elements into the atmosphere that can be analyzed to determine key details about the blast. One of the main points that intelligence officials want to know is whether the device was a plutonium bomb or one that used highly enriched uranium, which would be a first for North Korea.

In 2006 and 2009, North Korea is believed to have tested devices made of plutonium. But in 2010, Pyongyang revealed it was trying to enrich uranium, which would be a second source of nuclear bomb-making materials — a worrying development for the United States and its allies.

Generally, it takes about two days for such radioactive byproducts from the North's test site to reach South Korea, Defense Ministry spokesman Kim Min-seok said Wednesday.

Both South Korea and the Vienna-based Comprehensive Nuclear Test Ban Treaty Organization confirmed increased radiation levels following the North's 2006 nuclear test but didn't find anything in 2009. Experts in Seoul said the North plugged an underground testing tunnel in 2009 so tightly that no radioactive gas escaped.

The seismic event Tuesday was "roughly twice as big as what happened in 2009," Lassina Zerbo, head of CTBTO's international data center, said in a briefing. "The smoking gun will be the radio nuclides potentially released ... We cannot say anything about that before two or three days."

South Korea's Defense Ministry said Wednesday it has deployed cruise missiles with "world-class accuracy and destructive power" that are capable of hitting any target in North Korea at any time.

Tuesday's test, which set off powerful seismic waves that were measured using earthquake-detection sensors, drew immediate condemnation from Washington, the U.N. and others. Even North Korea's only major ally, China, summoned the North's ambassador for a dressing-down.

But the Obama administration's options for a response are limited, and a U.S. military strike is highly unlikely.

In an emergency session, the U.N. Security Council unanimously said the test poses "a clear threat to international peace and security" and pledged further action.

It remains to be seen, however, whether China will sign on to any new, binding global sanctions. Beijing, Pyongyang's primary trading partner, has resisted measures that would cut off North Korea's economy completely.

The test was a defiant North Korean response to U.N. orders that it shut down its 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.

Several U.N. resolutions bar North Korea from conducting nuclear or missile tests because the 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 it has 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.

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 U.N. Security Council recently punished North Korea for a rocket launch in December that the U.N. and Washington called a cover for a banned long-range missile test. Pyongyang said it was a peaceful launch of a satellite into space. In condemning that launch, the council 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.

The test came 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.

The test "was neither a surprise nor an occasion for panic," said Robert Hathaway, director of the Woodrow Wilson Center's Asia program. "Nonetheless, this latest provocation clearly constitutes a serious challenge to U.S. and international efforts to block the North from acquiring a nuclear weapons arsenal."

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 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.

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.

"North Korea will want to send a message that its nuclear and missile issues cannot be resolved with sanctions and that high-level talks with the U.S. are necessary," said Cheong Seong-chang, an analyst at the private Sejong Institute in South Korea.

He predicted U.S.-North Korea diplomatic talks could come later this year.

"The biggest U.S. concern is whether the North has made progress in its uranium enrichment program. It's a matter of nuclear proliferation. To resolve this, the U.S. cannot help but talking with North Korea," he said.

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Associated Press writers Sam Kim in Seoul, South Korea, Eric Talmadge in Tokyo, Christopher Bodeen in Beijing, George Jahn in Vienna, Bradley Klapper and Matthew Pennington in Washington, and Edith M. Lederer at the United Nations contributed to this report.