The United States says a rocket or nuclear test by North Korea would only isolate its communist government further.
State Department spokeswoman Victoria Nuland said Monday she was not in a position to confirm whether the North was preparing for its third nuclear test, in addition to a long-range rocket launch expected some time between April 12 and 16.
She urged China to use its influence with North Korea to dissuade it from going ahead with the launch.
Nuland said a launch would be "highly provocative" and a nuclear test "would be equally bad if not worse."
South Korean intelligence officials say recent satellite images show North Korea is digging a new underground tunnel in what appears to be preparation for a nuclear test.