SEOUL, South Korea -- In response to North Korea's latest nuclear test, South Korea on Thursday announced it would resume cross-border propaganda broadcasts that Pyongyang considers an act of war. Seoul also began talks with Washington that could see the arrival of nuclear-powered U.S. submarines and warplanes to the Korean Peninsula.
From Seoul to Washington, Beijing to the United Nations, world powers are looking at ways to punish Pyongyang for the test of what it called a new and powerful hydrogen bomb.
The loudspeaker broadcasts, which will start Friday, are certain to infuriate authoritarian Pyongyang because they are meant to raise questions in North Korean minds about the infallibility of the ruling Kim family. South Korea stopped earlier broadcasts after it agreed with Pyongyang in late August on a package of measures aimed at easing animosities that had the rivals threatening war.
Experts, meanwhile, are trying to uncover more details about the detonation that drew worldwide skepticism and condemnation.
It may take weeks or longer to confirm or refute the North's claim that it successfully tested a hydrogen bomb, which would mark a major and unanticipated advance for its still-limited nuclear arsenal. Even a test of an atomic bomb, a less sophisticated and less powerful weapon, would push its scientists and engineers closer to their goal of building a nuclear warhead small enough to place on a missile that can reach the U.S. mainland.
Statements from the White House said President Barack Obama had spoken to South Korean President Park Geun-Hye and to Prime Minister Shinzo Abe of Japan. The statements said the countries "agreed to work together to forge a united and strong international response to North Korea's latest reckless behavior."
Obama reaffirmed the "unshakeable U.S. commitment" to the security of South Korea and Japan, according to the statements.
Park's office said she also spoke with Abe over the phone and that they vowed cooperation to ensure that the U.N. Security Council imposes strong and effective measures against the North.
South Korean and U.S. military leaders also discussed the deployment of U.S. "strategic assets" in the wake of the North's test, Seoul's Defense Ministry said Thursday.
When animosities sharply rose in the spring of 2013 following North Korea's third nuclear test, the U.S. took the unusual step of sending its most powerful warplanes B-2 stealth bombers, F-22 stealth fighters and B-52 bombers -- to drills with South Korea in a show of force. B-2 and B-52 bombers are capable of delivering nuclear weapons.
The U.N. Security Council held an emergency session and pledged to swiftly pursue new sanctions against North Korea, saying its test was a 'clear violation' of previous U.N. resolutions.
Four rounds of U.N. sanctions have aimed at reining in the North's nuclear and missile development programs, but Pyongyang has ignored them and moved ahead to modernize its ballistic missiles and nuclear weapons.
When Seoul briefly restarted the loudspeaker broadcasts in August for the first time in 11 years in retaliation for land mine blasts blamed on Pyongyang that maimed two South Korean soldiers, the North threatened to attack the South.
In August, Seoul signed a package of agreements with Pyongyang on easing the standoff, which included a stop to broadcasts unless "abnormal" situation should occur again. Senior presidential official Cho Tae-yong said Thursday the broadcast will resume because the North's bomb test was a violation of the August agreements.
South Korea also said Thursday it will limit entry to a jointly run factory park in North Korea, the last major symbol of inter-Korean cooperation. The park's operation won't likely be affected much as the restriction will apply to clients, potential buyers and service providers from South Korea, rather than managers who commute to work with North Korean laborers.
North Korea said Wednesday it had successfully tested a "miniaturized" hydrogen bomb that elevated the country's "nuclear might to the next level."
But an early analysis by the U.S. government was "not consistent with the claims that the regime has made of a successful hydrogen bomb test," White House spokesman Josh Earnest said.
South Korea's spy service said it thought the estimated explosive yield from the blast was much smaller than what even a failed hydrogen bomb detonation would produce.
Some believe North Korea might have detonated a boosted fission bomb, a weapon considered halfway between an atomic bomb and an H-bomb.
But even if the North exploded a boosted fission bomb, its explosive yield, estimated at six kilotons, showed the test was likely a failure, a South Korean defense official said Thursday. An explosion two to five times more powerful would have been reported if it were successful, the official said, requesting anonymity because of department rules.
The North's 2013 test produced an estimated yield of 6-7 kilotons of explosives, according to South Korean officials.
Fusion is the main principle behind the hydrogen bomb, which can be hundreds of times more powerful than atomic bombs that use fission. In a hydrogen bomb, a nuclear fission explosion sets off a fusion reaction responsible for a powerful blast and radioactivity.
The hydrogen bomb already is the global standard for the five nations with the greatest nuclear capabilities: the U.S., Russia, France, the U.K. and China. Other nations may either have it or are working on it, despite a worldwide effort to contain such proliferation.
Just how big a threat North Korea's nuclear program poses is a mystery. North Korea is thought to have a handful of rudimentary nuclear bombs and has spent decades trying to perfect a multistage, long-range missile to carry smaller versions of those bombs.
Some analysts say the North probably hasn't achieved the technology needed to make a miniaturized warhead that could fit on a long-range missile capable of hitting the U.S. mainland. But debate is growing on just how far the North has advanced.
To build its nuclear program, the North must explode new and more advanced devices so scientists can improve their designs and technology. Nuclear-tipped missiles could then be used as deterrents and diplomatic bargaining chips -- especially against the U.S., which Pyongyang has long pushed to withdraw its troops from the region and to sign a peace treaty formally ending the Korean War.
U.S. aircraft designed to detect evidence of a nuclear test, such as radioactive particulate matter and blast-related noble gases, could be deployed from a U.S. base on the Japanese island of Okinawa. Japanese media said Tokyo mobilized its own reconnaissance aircraft over the Sea of Japan to try to collect atmospheric data.
Associated Press writers Kim Tong-hyung and Youkyung Lee contributed to this report.