South Korea to resume anti-North broadcasts after H-bomb test
South Korea has said it will retaliate for a North Korean nuclear test by resuming cross-border propaganda broadcasts which Pyongyang considers an act of war.
The South's presidential office made the announcement on Thursday, a day after the North claimed to have carried out its first hydrogen bomb test. The claim has been disputed by outside governments and experts.
The South stopped earlier broadcasts after it agreed with Pyongyang in late August on a package of measures aimed at easing animosities.
Seoul said the broadcasts will resume on Friday.
Meanwhile, experts are trying to uncover more details about the detonation which drew worldwide scepticism 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 US 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 behaviour".
Mr Obama also reaffirmed the "unshakeable US commitment" to the security of South Korea and Japan, according to the statements.
South Korean and US military leaders also discussed the deployment of US "strategic assets" in the wake of the North's test, Seoul's Defensce Ministry said.
Ministry officials refused to elaborate about what US military assets were under consideration, but they likely refer to B-52 bombers, F-22 stealth fighters and nuclear-powered submarines.
When animosities sharply rose in the spring of 2013 following North Korea's third nuclear test, the US 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.
South Korea also said it will limit entry to a jointly run factory park in North Korea, the last major symbol of inter-Korean co-operation. The park's operation is unlikely to 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 labourers.
The United Nations Security Council, meanwhile, held an emergency session and pledged to swiftly pursue new sanctions against North Korea, saying its test was a "clear violation" of previous UN resolutions.
Four rounds of UN sanctions have aimed at reining in the North's nuclear and missile development programmes, but Pyongyang has ignored them and moved ahead to modernise its ballistic missiles and nuclear weapons.
North Korea said on Wednesday that it had successfully tested a "miniaturised" hydrogen bomb which elevated the country's "nuclear might to the next level".
But an early analysis by the US 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 that which 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 6.0 kilotons, showed the test was probably a failure, Seoul's Defence Ministry said. An explosion two to five times more powerful would have been reported if it were successful, an official said.
After the North's 2013 test, a yield of 6-7 kilotons was estimated, 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 is already the global standard for the five nations with the greatest nuclear capabilities: the US, Russia, France, the UK 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 programme poses is a mystery. North Korea is thought to have a handful of rudimentary nuclear bombs and has spent decades trying to perfect a multi-stage, long-range missile to carry smaller versions of those bombs.
Some analysts say the North has probably not achieved the technology needed to make a miniaturised warhead that could fit on a long-range missile capable of hitting the US mainland. But debate is growing on just how far the North has advanced.
To build its nuclear programme, 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 US, which Pyongyang has long pushed to withdraw its troops from the region and to sign a peace treaty formally ending the Korean War.
US aircraft designed to detect evidence of a nuclear test, such as radioactive particulate matter and blast-related noble gases, could be deployed from a US base on the Japanese island of Okinawa. Japanese media said Tokyo mobilised its own reconnaissance aircraft over the Sea of Japan to try to collect atmospheric data.