BEIJING -North Korea claimed Wednesday that it had tested a "miniaturized" hydrogen bomb less than a month after the country's dictator boasted that the weapon had been developed.

The surprise announcement further complicates efforts to curb the Stalinist nation's move toward a working nuclear arsenal and will almost certainly lead to a push for new sanctions at the United Nations. The regime in Pyongyang detonated a hydrogen device for the first time local time, Korean Central News Agency said. The explosion was initially detected as a magnitude 5.1 earthquake by the U.S. Geological Survey. North Korea carried out the test “safely” and “perfectly,” , describing the detonation as an act of “self-defense” from its enemies. The regime “has confidently risen to the ranks of nuclear powers with hydrogen bombs by perfectly succeeding in the historic test of a hydrogen bomb,” Korean Central Television anchorwoman Ri Chun Hui said.

It’s the second test since Kim Jong UN became supreme leader four years ago and began consolidating power through a series of purges and provocations against South Korea. Kim has rebuffed U.S. and Chinese efforts to restart disarmament talks and has expanded the country’s nuclear arsenal that he says is the best defense against a U.S. invasion.

“A fourth nuclear test would allow for miniaturization and diversification of nuclear weapons so was an essential step for North Korea,” said Ahn Chan Il, who heads the World Institute for North Korea Studies in Seoul. “North Korea may be thinking it can open up for reforms only when it’s recognized as a nuclear power.”

The Korean won declined as much as 0.8 percent to 1,197.85 per dollar, the weakest level since September. The Kospi index of shares dropped 0.6 percent as makers of defense products such as Speco Co. surged. Ten-year bonds advanced, pushing their yield down one basis point to 2.05 percent.