SEOUL: The United States, South Korea and Japan agreed to launch a "united and strong" international response to North Korea's apparent fourth nuclear test, as experts scrambled on Thursday to find more details about the detonation that drew worldwide skepticism and condemnation.

It may take weeks or longer to confirm or contradict 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 a less sophisticated atomic bomb would push its scientists and engineers closer to their goal of building a warhead small enough to place on a missile that can reach the US mainland.

Separate 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 also reaffirmed the "unshakeable US commitment" to the security of South Korea and Japan, according to the statements.

The UN 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 UN resolutions.

Four rounds of UN 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.

South Korean officials said they were considering a variety of punitive measures. The options include restarting border propaganda broadcasts that Seoul halted after it agreed with Pyongyang in late August on a package of measures aimed at easing animosities, Defence Minister Han Min-koo told lawmakers Thursday.

On Wednesday, there was a burst of jubilation and pride in North Korea's capital of Pyongyang, where a TV anchor said the test of a "miniaturized" hydrogen bomb had been a "perfect success" that 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.

He added that nothing had happened in the last 24 hours to change Washington's assessment of Pyongyang's technical or military capabilities.

South Korea's spy agency told lawmakers that it thought the estimated explosive yield from the blast was much smaller than what even a failed hydrogen bomb detonation would produce.

Other analysts agreed with that assessment.

"I'm pretty skeptical," said Melissa Hanham, senior researcher at the James Martin Center for Nonproliferation Studies at the Middlebury Institute for International Studies in Monterey, California. "The seismic data indicates it would be very small for a hydrogen test."

While also noting the quake was likely too small for an H-bomb test, Jaiki Lee, a professor of nuclear engineering at Seoul's Hanyang University, said the North could have experimented with a "boosted" hybrid bomb that uses some nuclear fusion fuel along with more conventional uranium or plutonium fuel.

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.

Washington and nuclear experts have been skeptical of past North Korean claims about hydrogen bombs, which are much more powerful and much more difficult to make than atomic bombs.

North Korea's state media called the test a self-defence measure against a potential US attack: "The (country's) access to H-bomb of justice, standing against the US, the chieftain of aggression ... is the legitimate right of a sovereign state for self-defence and a very just step no one can slander."

The hydrogen bomb already is the global standard for the five nations with the greatest nuclear capabilities: the US, Russia, France, the UK and China. Other nations may also 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 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 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 US, which Pyongyang has long pushed to withdraw its troops from the region and to sign a peace treaty formally ending the Korean War.

It could be weeks before the true nature of the test is confirmed by outside experts, if they are able to do so at all.

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.