The White House says the U.S. government's early analysis of underground activity in North Korea "is not consistent" with that country's claim of having conducted a successful hydrogen bomb test.
Spokesman Josh Earnest also says nothing has happened to change the U.S. government's assessment of North Korea's technical or military capabilities.
He says the U.S. government is still doing the work that's needed to learn more about the nuclear test North Korea claims to have conducted successfully on Wednesday.
Pyongyang's announcement of a successful hydrogen bomb test would mark a major and unanticipated advance for its still-limited nuclear arsenal.
U.S. Defense Secretary Ash Carter spoke by phone Wednesday to his South Korean counterpart Han Min-Koo, and they agreed that a North Korean nuclear test would be an "unacceptable and irresponsible provocation," according to Carter's press secretary, Peter Cook.
Cook said Carter reaffirmed the United States' treaty commitment to defend South Korea, which he said includes "all aspects of the United States' extended deterrence" - an allusion to a longstanding U.S. promise to defend SouthKorea with nuclear weapons if necessary.
Meanwhile, Russia's U.N. ambassador says it would be going "too far" to say that Russia supports more sanctions against North Korea in response to the country's announcement of a new nuclear test.