TOKYO — The operator of Japan's crippled Fukushima nuclear plant said on Tuesday it believes about 300 metric tons of highly contaminated water has leaked from a storage tank designed to hold overflows from the site.

Japan's Nuclear Regulation Authority has classified the leak as a level 1 incident, the second lowest, on an international scale for radiological releases, a spokesman told Reuters 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, and the disaster was assigned the highest rating of 7.

Since then, the operator, Tokyo Electric Power Co, has been struggling to contain radioactive leaks to the growing alarm of neighboring countries. It has been criticized for its failure to prepare for the disaster and it has also been accused of covering up the extent of the problems at the plant.

A puddle that formed near the tank that leaked is emitting a radiation dose of 100 millisieverts an hour measured about 50 centimeters (1.6 feet) above the surface, Kyodo News reported, citing Tokyo Electric.

That is equivalent to the limit for accumulated exposure over five years for Japanese nuclear workers.

A spokesman for the operator, known as Tepco, 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 liter, 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.

Tokyo Electric uses a jerry-rigged system to wash water over the melted uranium fuel rods to keep them cool and stable. The water flows into basements that have been leaking since the disaster.

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 it does not believe the water that leaked from the storage tank, which is about 500 meters (550 yards) from the shore, has escaped into the ocean.

But only this month the company, after months of denial, admitted 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.