North Korea says conducted 'successful' H-bomb test

North Korea said Wednesday it had carried out a "successful" hydrogen bomb test, a claim that -- if true -- massively raises the stakes over the hermit state's banned nuclear programme.

International condemnation was swift with neighbours South Korea and Japan decrying a gross violation of UN Security Council resolutions, while the White House said it was still studying the precise nature of the apparent test and vowed to "respond appropriately".

"The republic's first hydrogen bomb test has been successfully performed at 10:00 am (0330 GMT)," North Korean state television announced.

"With the perfect success of our historic H-bomb, we have joined the rank of advanced nuclear states," it said, adding that the test was of a miniaturised device.

A hydrogen, or thermonuclear bomb, uses fusion in a chain reaction that results in a far more powerful explosion than the fission blast generated by uranium or plutonium alone.

Last month, North Korean leader Kim Jong-Un suggested Pyongyang had already developed a hydrogen bomb.

The claim was questioned by international experts and there was continued scepticism over Wednesday's test announcement.