About 300 tonnes of highly contaminated water has leaked from a storage tank at the crippled Fukushima nuclear power plant in Japan, the plant's operator says.

Japan's Nuclear Regulation Authority has classified the leak in the tank, which is designed to hold overflows from the site, as a level 1 incident, the second lowest rank on an international scale for radiological releases, a spokesman told the Reuters news agency on Tuesday.

It is the first time Japan has issued a so-called INES rating since three reactors melted down at Fukushima in March 2011, shortly after the plant was wrecked by a an earthquake and tsunami. At the time, the disaster was assigned the highest INES rating of 7.

Since then, the Tokyo Electric Power Company (TEPCO), the operator, has been struggling to contain radioactive leaks to the growing alarm of neighbouring countries.

A TEPCO spokesperson said the information on the dose rate did not come from the company, saying only the leaked water contains 80 million becquerels of radiation per litre, without putting it into perspective. 

A becquerel is a measure of the release of radioactive energy, while dose rate indicates how much radiation a person would receive standing near the source of radiation.

Highly contaminated excess water is pumped out and stored in steel tanks on elevated ground away from the reactors, which lie adjacent to the coast.

TEPCO said that it does not believe the water that leaked from the storage tank, which is about 500m from the shore, has escaped into the ocean.

Earlier this month, however, after months of denial, the company admitted that contaminated water escaping from basements and trenches closer to the coast is reaching the ocean, prompting the government to step up its involvement in the plant's cleanup.

A South Korean Foreign Ministry spokesman confirmed on Tuesday media reports that the country's government has asked Japanese officials to explain the leakage of contaminated water into the Pacific Ocean.

The Fukushima-Daiichi plant went into multiple meltdowns after the disaster, and tons of water are being used daily to keep its reactors under control. Decommissioning is expected to take decades.

Source: Agencies