WASHINGTON — North Korea declared on Tuesday that it had detonated its first hydrogen bomb.

The assertion, if true, would dramatically escalate the nuclear challenge from one of the world's most isolated and dangerous states.

In an announcement, North Korea said that the test had been a "complete success." But it was difficult to tell whether the statement was true. North Korea has made repeated claims about its nuclear capabilities that outside analysts have greeted with skepticism.

"This is the self-defensive measure we have to take to defend our right to live in the face of the nuclear threats and blackmail by the United States and to guarantee the security of the Korean Peninsula," a female North Korean announcer said, reading the statement on Central Television, the state-run network.

The North's announcement came about an hour after detection devices around the world had picked up a 5.1 seismic event along the country's northeast coast.

It may be weeks or longer before detectors sent aloft by the United States and other powers can determine what kind of test was conducted. Ned Price, a spokesman for the White House National Security Council, said in a statement that American officials "cannot confirm these claims at this time."

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South Korea's K-9 155-millimeter self-propelled howitzer was deployed on a field in the border city of Paju near the Demilitarized Zone dividing the two Koreas on Wednesday. Credit Yonhap, via Agence France-Presse — Getty Images

But he said the White House expected "North Korea to abide by its international obligations and commitments."

The tremors occurred at or near the Punggye-ri nuclear test site, where three previous tests have been conducted over the past nine years.