By Ju-min Park and Meeyoung Cho SEOUL (Reuters) - North Korea said it had successfully conducted a test of a miniaturized 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 (2030 ET) on Wednesday," North Korea's official KCNA news agency said.