The White House said initial analysis of North Korea's reported nuclear test is not consistent with claims of a successful hydrogen bomb.
The White said they are "continuing the work necessary to learn more" about the reported nuclear test.
It added any kind of nuclear test is "provocative and a flagrant violation" of UN Security Council resolutions.
The White House said it had a "rock solid commitment" to the safety of its allies Japan and South Korea.
The White House said there have been conversations with Chinese officials over the test and "this is an issue on which we do agree with China and that agreement is significant".
It said the US will be "collecting addition evidence" and analysis to learn more about the test.
A US military statement said the low-level flight over ally South Korea was in response to the North's 'recent provocative action'.
Tens of thousands of people danced and applauded in central Pyongyang, North Korea, in support of the country's reported H-bomb test.
Why is this latest development in North Korea's quest for nuclear power seen as potentially significant?