Media playback is unsupported on your device Media caption Jonathan Marcus assesses North Korea's nuclear capabilities in light of the latest test claims

The US, South Korea and Japan have said they will be united in their response to North Korea's claim to have successfully tested a hydrogen bomb.

North Korea said it carried out the test on Wednesday - if confirmed it would be its fourth nuclear test, and its first of the more powerful H-bomb.

The UN Security Council has also agreed to start drawing up new measures against North Korea.

But scepticism remains over whether the North really did conduct such a test.

Experts have said the seismic activity generated by the blast was not large enough for it to have been a full thermonuclear explosion.

The White House said President Barack Obama had spoken separately to South Korea's President Park Geun-Hye and Japan's Prime Minister Shinzo Abe.

They "agreed to work together to forge a united and strong international response to North Korea's latest reckless behaviour", it said in a statement.

Mr Abe told reporters: "We agreed that the provocative act by North Korea is unacceptable... We will deal with this situation in a firm manner through the cooperation with the United Nations Security Council."

He added that Japan may take unilateral action, saying it is "considering measures unique to our nation".

South Korea's presidential office said in a statement that Ms Park and Mr Obama had agreed to closely co-operate and that the international community "must make sure that North Korea pays the corresponding price" for the nuclear test, reported Yonhap news agency.

Image copyright AFP Image caption North Korea released an image it said showed Kim Jong-un signing the order for the test

The United Nations' Security Council held an emergency session on Wednesday and condemned the test claim as "a clear threat to international peace and security."

Japan's ambassador to the UN, Motohide Yoshikawa, called for a swift and robust new UN resolution, insisting: "The authority and credibility of the Security Council will be put in question if it does not take these measures."

But the UN ambassador for Russia, which has been developing warmer relations with Pyongyang, said it would be going "too far" to say Moscow supported further sanctions.

North Korea's nuclear test

Image copyright AFP Image caption The note signed by Kim Jong-un authorising the test

Can North Korea now launch a nuclear missile?

Despite North Korea's claims, experts are sceptical that North Korea can make a nuclear weapon small enough to fit on a missile.

What do we know about the latest test?

Observers agree a nuclear explosion of some kind took place and it seems to have been a bit bigger than the last test in 2013, but not nearly big enough to be a full thermonuclear explosion - an "H-bomb" - as Pyongyang claims.

Why can't the world stop North Korea?

North Korea has a determination to defy both world opinion and heavy sanctions to reach its nuclear goal. Crucially, its main ally, China, has proved either unwilling or unable to help.

Meanwhile, South Korea has begun limiting entry to the Kaesong industrial park in North Korea, jointly run by both countries. Only those directly involved in operations there will be allowed to enter from the South, said Seoul's Unification Ministry.

Seoul has also said it will restart propaganda broadcasts across the border on Friday, an act which the North strongly opposes. The broadcasts were halted last year as part of a deal with the North to ease tensions that had escalated sharply in the summer.

The nuclear test came days before Kim Jong-un's 33rd birthday, which falls on Friday and is expected to be marked by celebrations.

'Bang would have been bigger'

Hydrogen bombs are more powerful and technologically advanced than atomic weapons, using fusion - the merging of atoms - to unleash massive amounts of energy.

Atomic bombs, like those that devastated two Japanese cities in World War Two, use fission, or the splitting of atoms.

Media playback is unsupported on your device Media caption North Koreans reacting to the news: "Developing a hydrogen bomb is inevitable"

Bruce Bennett, an analyst with the Rand Corporation, was among those casting doubts on Pyongyang's test: "The bang they should have gotten would have been 10 times greater than what they're claiming."

South Korea's intelligence agency also told politicians that the estimated power of the blast fell far short of what would be expected from a hydrogen bomb.

US White House spokesman Josh Earnest said "initial analysis was not consistent with North Korea's claims of a successful hydrogen bomb test". But he added: "Nothing that has occurred in the last 24 hours has caused the United States government to change our assessment of North Korea's technical and military capabilities."

Some analysts have suggested it is possible Pyongyang tested a "boosted" atomic bomb, which uses some fusion fuel to increase the yield of the fission reaction.

The US and nearby countries are thought to be carrying out atmospheric sampling, hoping to find leaked radioactive material, which would give clues as to what kind of device was tested.