SEOUL - North Korea said it successfully tested a miniaturized hydrogen nuclear bomb on Wednesday, claiming a significant advance in its strike capability and setting off alarm bells in Japan and South Korea.

The test, the fourth time the isolated state has exploded a nuclear device, was ordered by young leader Kim Jong Un and successfully conducted at 10:00 a.m. local time (8.30 p.m. ET on Tuesday), North Korea's official KCNA news agency said.

"Let the world look up to the strong, self-reliant nuclear-armed state," Kim wrote in what North Korean state TV displayed as a handwritten note.

South Korean intelligence officials and several analysts questioned whether Wednesday's explosion was indeed a full-fledged test of a hydrogen device.

But the reported nuclear test drew condemnation abroad, including from China and Russia, North Korea's two main allies.

China expressed "resolute opposition" and said it would lodge a protest with Pyongyang.

No countries were given advance warning of a nuclear test, South Korea's intelligence service said, according to lawmakers briefed by intelligence officials.

In previous such tests, Pyongyang had notified China, Russia and the United States beforehand, they said. While a fourth nuclear test had been long expected, the claim that it was a hydrogen device, much more powerful than an atomic bomb, came as a surprise, as did the timing.(Reuters)