National broadcaster NHK reported that water containing radioactive material leaked from the plant into the Sea of Japan, but that the radioactivity level was low and posed no environmental danger.
The reactor automatically shut down at the time of the leak, the report said. The quake triggered a fire at an electrical transformer at the plant, but plant operator Tokyo Electric Power Co. said earlier in the day that the reactor was not damaged.
The quake, which left fissures a metre wide in the ground along the coast, hit shortly after 10 a.m. local time and was centered off Niigata state. Buildings swayed 320 kilometres away in Tokyo. Sirens wailed in Kashiwazaki, a city of about 90,000, which appeared to be hardest hit. Japan's Meteorological Agency measured the quake at a 6.8 magnitude. The U.S. Geological Survey, which monitors quakes around the world, said it registered 6.7.
"I was so scared the violent shaking went on for 20 seconds," Ritei Wakatsuki, who was on her job in a convenience store in Kashiwazaki. "I almost fainted by the fear of shaking.''
Flames and billows of black smoke poured from the Kashiwazaki nuclear plant the world's largest in terms of power output capacity. The fire, at an electrical transformer, was put out shortly after noon and there was no release of radioactivity or damage to the reactors, Motoyasu Tamaki, a Tokyo Electric Power Co. official, said before the NHK report was released. Tsunami warnings were issued along the coast of Niigata but later lifted.
A series of smaller aftershocks rattled the area, including one with a 5.8 magnitude. The Meteorological Agency warned that the aftershocks could continue for a week.
The quake hit on Marine Day, a national holiday in Japan, when most people would have been at home.
Four women and three men all either in their 70s or 80s were killed, according to the National Police Agency in Tokyo and NHK, which reported more than 800 people were hurt.
Nearly 300 homes in Kashiwazaki a city known mainly for its fishing industry were destroyed and some 2,000 people evacuated, officials said.
A ceiling collapsed in a gym in Kashiwazaki where about 200 people had gathered for a badminton tournament, and one person was hurt, Kyodo reported. The quake also knocked a train car off the rails while it was stopped at a station. No one was injured.
Several bullet train services linking Tokyo to northern and northwestern Japan were suspended.
Prime Minister Shinzo Abe whose ruling party is trailing in the polls interrupted a campaign stop in southern Japan for upcoming parliamentary elections, rushed back to Tokyo and announced he would head to the damaged area. He later arrived in a blue uniform to survey the damage.
"Many people told me they want to return to their normal lives as quickly as possible," Abe told reporters in Kashiwazaki. "The government will make every effort to help with recovery."
Japan sits atop four tectonic plates and is one of the world's most earthquake-prone countries. The last major quake to hit the capital, Tokyo, killed some 142,000 people in 1923, and experts say the capital has a 90 per cent chance of suffering a major quake in the next 50 years.
In October 2004, a magnitude-6.8 earthquake hit Niigata, killing 40 people and damaging more than 6,000 homes. It was the deadliest to hit Japan since 1995, when a magnitude-7.2 quake killed 6,433 people in the western city of Kobe.