U.S., experts cast doubt on North Korea's H-bomb claim

North Korea said it successfully tested a powerful nuclear bomb on Wednesday, drawing criticism from world powers even though experts and the U.S. government doubt that the isolated nation's atomic weapons capability is as advanced as Pyongyang claims. 

It was the fourth time that North Korea has exploded a nuclear device. It unnerved neighbors South Korea and Japan and prompted an emergency meeting on Wednesday of the U.N. Security Council in New York.

While a nuclear test had long been expected, North Korea's assertion that it exploded a hydrogen device, much more powerful than an atomic bomb, came as a surprise. The White House said North Korea might not in fact have tested a hydrogen bomb.

The explosion caused an earthquake that was measured by the United States Geological Survey.

Pyongyang also said it was capable of miniaturizing, allowing a nuclear device to be adapted as a weapon and placed on a missile, potentially posing a new threat to the United States and its allies in Asia.

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

While the Kim government boasts of its military might to project strength globally, it also plays up the need to defend itself from external threats as a way to maintain control domestically.

The test drew world criticism, including from China and Russia. China, the major trade partner of North Korea, said it will lodge a protest with Pyongyang.

White House spokesman Josh Earnest said any nuclear test would be a "flagrant violation" of U.N. Security Council resolutions. "The initial analysis is not consistent with the claim the regime has made of a successful hydrogen bomb test," he told reporters.

Conventional atomic bombs split atoms from heavier elements such as uranium or plutonium. They occur in one stage. The process is called fission. Hydrogen bombs have a second stage after fission. This fusion stage releases much more energy.

North Korea has been under U.N. Security Council sanctions since it first tested an atomic device in 2006 and could face additional measures.

The Security Council said it would begin working immediately on significant new measures in response to North Korea, a threat diplomats said could mean an expansion of U.N. sanctions against Pyongyang.