"We have confirmed that water containing a slight amount of radioactive materials leaked out of the facility," said Shougo Fukuda, a spokesman for operator Tokyo Electric.

"But the leakage is believed to be far below the levels that could affect the environment," he said, adding that no workers at the plant were exposed to radiation.

Black smoke billowed for hours from the plant until the blaze was extinguished.

The fire took place in electricity supply facilities and not the nuclear reactors themselves, which were automatically shut down by well-established quake safety measures, Tokyo Electric said.

Monday's earthquake reduced homes to rubble in the areas hardest hit northwest of the capital Tokyo and triggered small tsunami waves.

At least seven people were killed and hundreds more injured in the quake, which was followed by dozens of aftershocks.

Japan, which has the world's second largest economy but virtually no natural energy resources, relies on nuclear plants for close to 35 percent of its power and aims to boost the figure to 40 percent by 2010.

But the country also endures about 20 percent of the world's major earthquakes, posing a challenge for nuclear power operators.

In recent years, a string of accidents at nuclear plants across Japan has triggered fears among the general public and opposition about the construction of new facilities.

Japan is also particularly sensitive about nuclear accidents as it is the only country to have suffered atomic attack.

The Kashiwazaki-Kariwa facility has seven reactors, four of which were in operation Monday before the quake. The plant has a total capacity of 8,212 megawatts supplying Tokyo, the world's largest metropolis.

Japan currently has 57 nuclear power plants operating and they are subject to strict safety controls.

As demonstrated Monday, all nuclear plants in Japan are designed to stop automatically when they detect earthquakes.

The facilities are supposed to be built on rock grounding and those alongside the sea are also protected by anti-tsunami walls.

Japan updated its regulations on nuclear power plants after a massive 1995 earthquake of 7.2 on the Richter scale devastated the western city of Kobe, killing more than 6,400 people.

Since then, all nuclear reactors are required to withstand earthquakes of 7.75 on the Richter scale.

In regions seen as particularly vulnerable to quakes, such as in the northern town of Rokkasho where a nuclear reprocessing plant is undergoing tests, facilities are supposed to resist quakes up to 8.25 on the Richter scale.

In March 2006, a Japanese court for the first time ordered a nuclear reactor to shut down after nearby residents complained that it did not sufficiently meet anti-quake standards.

But the plant in the central town of Shika is still operating as its operator, Hokuriku Electric Power, has lodged an appeal.

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