Claims by North Korea that it conducted a hydrogen bomb test have been met by worldwide condemnation.
The secretive state said it successfully tested a miniaturised hydrogen bomb, a move which would be a significant advancement of its nuclear armoury.
It was called a "provocation" by Foreign Secretary Philip Hammond, while the Japanese Prime Minister said he "cannot allow this".
But both the South Korean spy agency and a number of weapons experts cast doubt on whether it was an H-bomb being tested.
The magnitude of the earthquake following the explosion was not consistent with what would be expected from such a blast, critics said.
North Korean state media said they had developed the "H-bomb of justice" as a response to perceived US aggression.
It said: "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 test, the state's first since 2013, came as a surprise to many, as no mention of it had been made in leader Kim Jong-un's New Year speech.
Experts believe the bomb will largely be used as a means of increasing North Korea's influence on the world stage.
The US Geological Survey measured an earthquake with a magnitude of 5.1 - bigger than the three previous bombs in 2013, 2009 and 2006.
But the explosive yield is only a fraction of what would be expected during the explosion of a hydrogen bomb, it is claimed.
South Korean politician Lee Cheol Woo said he had been briefed by the country's National Intelligence Service and was told even a failed H-bomb detonation would have a higher explosive yield than was registered.
Chemical weapons analyst Karl Dewey, of IHS Jane's, suggested it was more likely the blast was caused by a boosted fissure weapon.
He said: "Hydrogen bombs use lithium deuteride and it is not known if North Korea has the infrastructure to create such material.
"What may be more plausible is the development of what is known as a boosted fission weapon. Simple fission weapons or boosted weapons can be used to set off a thermonuclear secondary, but shouldn't be confused with them."
Although the development of an H-bomb would be an enormous acceleration in North Korea's nuclear capabilities, it still lacks the means to launch them long-distance.
The state military has yet to perfect the multi-stage ballistic missiles which are required for an intercontinental launch.
South Korea led the calls for its rival to be hit with further sanctions by the UN.
Japanese Prime Minister Shinzo Abe said of the bomb test: "We absolutely cannot allow this, and condemn it strongly."
The Foreign Secretary tweeted: "If North Korean H-bomb test reports are true, it is a grave breach of UNSC resolutions & a provocation which I condemn without reservation."
Mr Hammond will discuss the developments with his Japanese and South Korean counterparts.
UN Security Council members were meeting in New York on Wednesday to discuss the North Korean claims.
A Downing Street spokeswoman said the UK's "initial assessment" confirmed that Pyongyang had carried out a nuclear test, adding: "As to whether or not this is an H-bomb, we need to look at that more."
The spokeswoman added: "This is clearly concerning and would be a breach of UN Security Council resolutions."