PYONGYANG, North Korea — Defying U.N. warnings, North Korea on Tuesday conducted its third nuclear test in the remote, snowy northeast, taking a crucial step toward its goal of building a bomb small enough to be fitted on a missile capable of striking the United States.

North Korea said the atomic test was merely its "first response" to what it called U.S. threats, and said it will continue with unspecified "second and third measures of greater intensity" if Washington maintains its hostility.

The underground test, which set off powerful seismic waves, drew immediate condemnation from Washington, the U.N. and others. Even its only major ally, China, voiced opposition.