In North Korea: Government says miniaturised hydrogen nuclear test successful

Some analysts questioned whether Wednesday's test was indeed of a hydrogen device.

North Korea said it had successfully conducted a test of a miniaturised hydrogen nuclear device on Wednesday morning, marking a significant advance in the isolated state's strike capabilities and raising alarm bells in Japan and South Korea.

The test, the fourth time North Korea has exploded a nuclear device, was ordered by young leader Kim Jong Un, state media said.

"The first H-bomb test was successfully conducted at 10:00 (0130 GMT) on Wednesday," North Korea's official KCNA news agency said.

Last month, Kim appeared to claim his country had developed a hydrogen bomb, also known as a thermonuclear device, a step up from the less powerful atomic bomb, but the United States and outside experts were sceptical at the time.

Some analysts questioned whether Wednesday's test was indeed of a hydrogen device.

"North Korea has made claims about its nuclear and missile programs in the past that simply have not held up to investigation," said Melissa Hanham, a Senior Research Associate at the California-based James Martin Center for Nonproliferation Studies.

Yang Uk, a senior research fellow at the Korea Defence and Security Forum, added: "Given the scale it is hard to believe this is a real hydrogen bomb. They could have tested some middle stage kind (of device) between an A-bomb and H-bomb, but unless they come up with any clear evidence, it is difficult to trust their claim."

The United States Geological Survey reported a 5.1 magnitude quake that South Korea said was 49 km (30 miles) from the Punggye-ri site where the North has conducted nuclear tests in the past.

North Korea's last test, of an atomic device in 2013, also registered at 5.1 on the USGS scale.

The claim of miniaturising, which would allow the device to be adapted as a weapon and placed on a missile, would pose a new threat to the United States and its regional allies, Japan and South Korea.