North Korea said it successfully tested a miniaturized hydrogen nuclear device 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 a.m. local time  North Korea's official KCNA news agency said.

RELATED: Aide to North Korea's Kim Jong Un dies in sudden car crash

"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.

The nuclear test drew condemnation abroad, with China, the North's chief ally, expressing "resolute opposition" and saying it would lodge a protest with Pyongyang.

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. It ensures that North Korea will be a key topic during the U.S. presidential campaign.

RELATED:  North Korea girl band show gets cancelled over 'anti-American lyrics': Source

North Korea has long coveted diplomatic recognition from Washington but sees its nuclear deterrent as crucial to ensuring the survival of its third-generation dictatorship.

"With Iran being off the table, the North Koreans have placed themselves at the top of the foreign policy agenda as far as nation-states who present a threat to the U.S.," said Michael Madden, an expert on the country's secretive leadership.

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

RELATED: North Korea releases NYU student after six-month detainment

The device had a yield of about 6 kilotons, according to the office of a South Korean lawmaker on the parliamentary intelligence committee – roughly the same size as the North's last test, which was equivalent to 6-7 kilotons of TNT.

"Given the scale, it is hard to believe this is a real hydrogen bomb," said Yang Uk, a senior research fellow at the Korea Defence and Security Forum.

"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."