Rescue workers in northwestern Japan are continuing their search for survivors of a powerful earthquake that killed nine elderly people and caused a radiation leak at a nuclear plant.

Officials at the Kashiwazaki Kariwa plant say the 6.8 magnitude quake Monday morning triggered a fire at the site, which led to water containing radioactive material to leak into the Sea of Japan. But they say the water's radioactivity level is within safety limits.

Authorities say as many as a thousand people were injured in the tremor just off the coast of Niigata prefecture, and hundreds of buildings collapsed. Thousands of people spent the night in emergency centers.

The area was rattled by a series of aftershocks all day. Late Monday (local time), another strong earthquake measuring 6.6 struck off Japan's west coast, but there were no immediate reports of fresh damage.

Japanese Prime Minister Shinzo Abe cut short campaigning for parliamentary elections to inspect the damage.

Niigata prefecture's governor has requested help from Japan's Self Defense Forces.

Tens of thousands of people are without electricity and other utilities, and wet weather is raising fears of mudslides in the area, where soil is already oversaturated from a recent typhoon.