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 Obama used his State of the Union address to assure U.S. allies in the region and warn 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 was unclear to outsiders.
Intelligence officials and analysts in Seoul raised the possibility of another nuclear test and of ballistic missile test-launches. North Korea's Foreign Ministry said the latest test was merely its "first response" to what it called U.S. threats and that 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 it used aircraft and ships, as well as specialists on the ground, to collect air samples to analyze possibly increased radiation from the test, according to the 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.
South Korea also 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, and is speeding up the planned deployment of ballistic missiles.
Kim said Seoul believes North Korea made test preparations at two underground tunnels and may still be able to conduct another atomic test in the second unused tunnel.
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.
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."
Tuesday's test was North Korea's first since young leader Kim Jong Un took power over 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.
It wasn't immediately clear to outside experts whether the device exploded Tuesday was small enough to fit on a missile. A successful test would take North Korean scientists a step closer to building a nuclear warhead that could 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 them 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, referring to the possibility of another nuclear or missile test.
Despite tensions, he predicted U.S.-North Korea diplomatic talks could occur 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 talk with North Korea," he said.
Associated Press writers Sam Kim in Seoul, South Korea, Eric Talmadge in Tokyo and Edith M. Lederer at the United Nations contributed to this report.