Rescue teams searched through matchstick rubble Saturday for thousands of people missing in flooded areas of northeastern Japan, beginning one of the most complex relief efforts in history.

Japanese authorities said Sunday that efforts to restart the cooling system at one of the reactors damaged by Friday's earthquake had failed, a major setback in the struggle to contain what has become the most serious nuclear power crisis in a quarter century.

An explosion rocked one of Japan's nuclear power plants, causing a portion of a building to crumble, sending white smoke billowing into the air and prompting Japanese officials to warn those in the vicinity to cover their mouths and stay indoors.

A bulldozing tsunami triggered by an 8.9-magnitude earthquake devastated the northeast coast of Japan on Friday, turning cars into driftwood, washing away neighborhoods and leaving this industrialized country bracing for an epic humanitarian disaster.

More than 50 reported dead after waves as tall as 20 feet hit buildings, vehicles; tsunami warnings issued for a dozen countries, including the U.S.