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 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 programmes 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 from the Punggye-ri site where the North has conducted nuclear tests in the past, according to a News agency.