WASHINGTON -- The White House says initial evidence shows that North Korea was not able to successfully test a hydrogen bomb, as Pyongyang has claimed.
The comments by spokesman Josh Earnest on January 6 follow broad international condemnation about the test and wide speculation about what exactly it involved.
International seismic monitors detected the test early on January 6 in a northeastern region of North Korea and the government announced on state TV that a thermonuclear weapon had been successfully tested.
Earnest told reporters at the White House that there had been some sort of nuclear test.
However, he said, "the initial analysis...is not consistent with North Korean claims of a successful hydrogen-bomb test."
"Any type of nuclear test, like the one North Korea conducted last night is provocative and a flagrant violation of multiple United Nations resolutions," Earnest said.
"What is true is that North Korea continues to be one of the most isolated nations in the world, and their isolation has only deepened as they have sought to engage in increasingly provocative acts," he added.
South Korea's spy agency said the estimated explosive yield from the North Korean test was much smaller than what would result even from a failed hydrogen-bomb blast.
Despite four United Nations Security Council resolutions passed since 2006 and other sanctions, Washington and its allies in East Asia and elsewhere have watched helplessly as North Korea has continued to develop its nuclear arsenal as well as its missile technology.
Pyongyang has conducted at least four underground nuclear tests over the past 20 years -- three during President Barack Obama's administration -- and it is the only country to have conducted any tests in the past 15 years, according to the White House.
A hydrogen, or thermonuclear, weapon is a substantially more powerful weapon than regular fission bombs of the sort that were dropped on Hiroshima and Nagasaki at the end of World War II. The United States, Russia, China, Britain, and France are all known to possess hydrogen weapons.
Analysis conducted the Institute for Science and International Security and other experts estimate North Korea has stockpiled enough enriched uranium or plutonium to build fewer than 30 nuclear weapons.
After holding emergency, closed-door meetings, the UN Security Council unanimously condemned the test, calling it a "clear violation" and "therefore a clear threat to international peace and security."
The United States has about 28,500 military personnel stationed in South Korea.