A strong earthquake has hit northwestern Japan, killing at least seven people and causing a small radiation leak at a nuclear power plant.

Officials at the Kashiwazaki Kariwa plant say the earthquake sparked a fire at the site, which led to water containing radioactive material leaking into the Sea of Japan. They say the radioactivity level of the water is within safety levels and poses no danger to the environment.

About 800 people in northwestern Japan are suffering from injuries from the 6.8 magnitude quake that struck Monday  just off the coast, southwest of Niigata City.

The earthquake and its aftershocks caused hundreds of buildings to collapse and made buildings sway in Tokyo, 250 kilometers away.

Japanese Prime Minister Shinzo Abe has interrupted a campaign stop to visit the disaster zones.
Niigata prefecture's governor has requested help from Japan's Self Defense Forces.

Tens of thousands of people have lost access to utilities, including gas, electricity and water.

Earthquakes are common in Japan because the country lies at the junction of four of the tectonic plates that move just beneath the earth's surface.  About 20 percent of the world's most powerful earthquakes take place in the region.

Some information for this report was provided by AFP and AP.