SEOUL—North Korea said it exploded a nuclear device Tuesday, its third experimental detonation in a long effort to build weapons of mass destruction that the U.S. and other countries consider a serious threat.

"A third nuclear test has been successfully staged," said a statement from North Korea's state news agency that was published about three hours after seismic experts in South Korea, the U.S. and Japan announced they had detected an "artificial earthquake" in the country.

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