A strong earthquake flattened houses in northwestern Japan on Monday (July 16), killing at least seven people, forcing the evacuation of thousands and sparking a small fire at the world's biggest nuclear power plant. It was later revealed water containing radioactive materials had been leaked into the sea. More than 800 people were injured by the quake, measured at 6.8 on the Richter scale, which centred about 60 km (37 miles) southwest of Niigata. Buildings swayed in Tokyo, 250 km (155 miles) away. Nuclear reactors at the plant in Niigata prefecture automatically shut down for checks. The operators Tokyo Electric Power Co (TEPCO) said contaminated water that leaked from the Kashiwazaki-Kariwa plant had not had any environmental impact. Two women in their 80s died when their houses collapsed during the quake. Police said the other known victims were also elderly and a 77-year-old man is missing after going for a walk before the quake hit at 10:13 a.m. (0113 GMT). Streets were littered with damaged street signs and debris, while rescuers struggled to find survivors from collapsed buildings. Troops and extra emergency teams were being sent to help with rescue and relief efforts. Houses, many wooden with traditional heavy tile roofs, were flattened, a temple roof caved in and roads cracked in the quake, which was centred in the same northwestern area as a tremor three years ago that killed some 65 people. Tsunami warning sirens sounded along affected stretches of the Sea of Japan, but the alert was later withdrawn. Bullet trains stopped services in northern Japan after the quake, and again after the largest aftershock. A local train toppled from the rails, but media said no one was injured. Landslides closed several local roads, and rain was forecast in the area for the next two days, raising worries about more slippage. Japanese Prime Minister Shinzo Abe cut short campaigning for parliamentary elections. He told reporters that he was on his way to visit the area and wanted to reassure people there that help was on its way. A fire in an electrical transformer at the Kashiwazaki Kariwa nuclear power plant -- the world's largest -- was quickly extinguished. Tokyo Electric Power Co. (TEPCO) said 1.5 litres of water containing radioactive materials had leaked from the No. 6 unit at its Kashiwazaki-Kariwa nuclear power plant. The company said in a statement that the contaminated water had been released into the ocean and had had no effect on the environment. TEPCO said earlier there were no radiation leaks at the plant, where reactors automatically shut down for checks. Abe's government set up an emergency office to deal with the quake, which officials said had damaged about 500 buildings. The quake halted gas service to about 35,000 homes and disrupted the water supply to all of Kashiwazaki, a city with a population of around 95,000 that was hardest hit by the quake, media and officials said. About 25,000 homes in Niigata prefecture were without electricity, a local official said. About 7,800 people had fled their homes to nearly 100 evacuation centres as scores of aftershocks of up to magnitude 5.6 rattled the area, state broadcaster NHK said. Hundreds of houses were totally or partially destroyed by the quake, sending survivors into local public halls and school gymnasiums that were turned into make-shift evacuation shelters. "My house was totally destroyed and the only thing I could think of was escaping and taking my children to safety," Itsuko Igarashi, a local resident and a mother of two, said at an evacuation centre. Her six-year-daughter daughter Sae Igarashi added: "I was shaking and frightened." Japan is one of the world's most earthquake-prone countries, with a tremor occurring at least every five minutes.