A 6.8-magnitude earthquake rocked Japan's north-west coast yesterday, killing at least seven people, injuring hundreds and causing a nuclear reactor to leak water containing radioactive material into the sea.

The Kashiwazaki Kariwa nuclear power plant, the world's largest in terms of power output capacity, leaked about 1.2 cubic metres - or 1200 litres - of water in the building housing one of its seven reactors, said Katsuya Uchino, an official with plant operator Tokyo Electric Power.

Uchino said the water contained a tiny amount of radioactive material - a billionth of the guideline under Japanese law - and is believed to have flushed into the sea.

A company statement said the leak had stopped and that there had been no "significant change" in the sea water under surveillance and no effect on the environment.

The reactor had been shut down at the time of the leak. The quake triggered a fire at an electrical transformer at the plant, but Tokyo Electric Power Co said earlier in the day the reactor had not been damaged.

In Kashiwazaki city, the quake reduced older buildings to piles of timber. Kyodo News agency reported more than 900 people were hurt, with injuries including broken bones, cuts and bruises, and some 780 buildings sustained damage, more than 300 of them destroyed.

Seven people in their 70s and 80s - four women and three men - died in the quake, most after being crushed when buildings collapsed on them, said officials with the National Police Agency in Tokyo.

"I was so scared - the violent shaking went on for 20 seconds," Ritei Wakatsuki, an employee of a convenience store, said from Kashiwazaki. "I almost fainted by the fear of shaking."

The area was plagued by a series of aftershocks. There were no immediate reports of additional damage or injuries from the aftershocks, which triggered no tsunami warnings.

Yesterday evening, a 6.6-magnitude quake hit off Japan's west coast, shaking wide areas of the country, but it was unrelated to the Niigata quakes. There were no immediate reports of fresh damage.

Around 10,000 people in the quake zone were evacuated from their homes, according to media reports. Getting food and water supplies to evacuation centres was a top concern, governor Hirohiko Iizumida told reporters. Rescue workers dug through flattened buildings in hopes of pulling victims alive from the wreckage.

The force of the quake buckled seaside roads and bridges, and one-yard wide fissures could been seen in the ground along the coastline. The quake also knocked a train car off the rails while it was stopped at a station. No-one was injured.

The Meteorological Agency issued tsunami warnings along the coast of Niigata, but the warnings were later lifted.

The quake, which hit the region at 10.13am local time was centred off the coast of Niigata, 160 miles north-west of the capital, Tokyo.

The tremor made buildings in Tokyo sway and was also felt in northern and central Japan.-AP