Washington: The White House on Wednesday rejected North Korea's claim of having successfully tested a hydrogen bomb for the first time.
"The initial analysis that has been conducted ... is not consistent with North Korea's claim of a successful hydrogen bomb test," White House spokesman Josh Earnest said.
"There is nothing that has occurred in the last 24 hours that has caused the United States government to change our assessment of North Korea's technical and military capabilities," he added.
The North Korean claim was broadcast on state television in Pyongyang, which declared that the republic's first hydrogen bomb test" had been "successfully performed at 10:00 am (0130 GMT)."
"We have now joined the rank of advanced nuclear states," it said, adding that the test was of a miniaturized device.
Earnest said US President Barack Obama would speak later in the day with Japanese Prime Minister Shinzo Abe and South Korean President Park Geun-Hye.
US Secretary of State John Kerry meanwhile said he had been in contact with his counterparts, reaffirming US security commitments to its allies in East Asia.
"This highly provocative act poses a grave threat to international peace and security and blatantly violates multiple UN Security Council resolutions," he said in a statement.
"We do not and will not accept North Korea as a nuclear armed state, and actions such as this latest test only strengthen our resolve," he said.
The United States will continue to work closely with the UN Security Council and countries that have engaged North Korea in nuclear talks "to take appropriate action," Kerry added, without elaborating on what that might be.
The UN Security Council was considering further sanctions against North Korea, which has conducted three previous nuclear tests since 2006 in defiance of UN resolutions.