Black smoke rises from one of Kashiwazaki-Kariwa nuclear power plant’s four reactors in northwestern Japan on Monday. (AP)

Kashiwazaki (Japan), July 16 (Reuters): A strong earthquake flattened houses in northwestern Japan today, killing at least seven people, and sparking a small radiation leak and fire at the world’s biggest nuclear power plant.

According to the US Geological Survey, a 6.8 magnitude earthquake struck off the Japanese coast hours after a quake of similar strength hit northwestern Japan.

More than 800 people were injured by the quake in Niigata prefecture and buildings swayed as far away as Tokyo. Thousands were evacuated from their homes.

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 world’s largest.

The company said 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.

Two women in their 80s died when their houses collapsed during the magnitude 6.8 tremor. A police spokesman confirmed the deaths of seven elderly people, and a 77-year-old man was reported missing after going for a walk before the quake hit at 0113 GMT.

“First there was a sharp vertical jolt and then it shook sideways for a long time and I couldn’t stand up. Tall shelves fell over and things flew around,” said Harumi Mikami, 55, a teacher who was at her school in Kashiwazaki City, near the focus of the quake in Niigata prefecture about 250 km northwest of Tokyo.

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.

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.