The operator of Japan's wrecked Fukushima nuclear power plant was "careless" in monitoring tanks containing dangerously radioactive water, the country's nuclear regulator has said.

The Tokyo Electric Power Company (TEPCO) also failed to keep records of inspections of the tanks, Nuclear Regulation Authority (NRA) Commissioner Toyoshi Fuketa told reporters after a visit to the nearby Fukushima Daiichi plant on Friday.

Fuketa visited the plant after NRA chairman Shunichi Tanaka said this week he was concerned more of the hastily built giant containers would fail.

"Fundamentally, for a facility holding that kind of radioactive water, they did not take action that foresaw the risks of possible leaks," Fuketa said.

"On top of that, and this is an impression I had before my visit, I can't help but say that the inspections were careless."

Serious breach

Japan's nuclear crisis this week escalated to its worst level since a massive earthquake and tsunami crippled the plant more than two years ago, with TEPCO saying a tank holding highly contaminated water leaked 300 tonnes (300,000 litres) of radioactive fluid.

It was the fifth and most serious breach of the same type of tank, as the crisis goes from bad to worse, prompting neighbouring China to express shock at the continuing leaks.

TEPCO crews worked to gather as much of the water as possible from the area where it leaked, they were not able to stop its absorption into the soil.

Groundwater that has mixed with polluted water has already seeped into the ocean, with TEPCO launching an operation to pump it out of 28 wells, the company said on Friday.

In response to the leak, Fukushima's fishing associations has suspended all fishing off the coast as of September, according to the Japanese Kyodo news agency.

New spots of radiation

There are hundreds of tanks in place at Fukushima holding radioactive water used to cool the fuel rods. The plant is fast running out of space.

TEPCO said on Thursday that new spots of high radiation had been found near the storage tanks, raising fear of fresh leaks.

Tokyo Electric, which has long had problems with documentation, did not keep proper records of its tank inspections and therefore missed problems, Fuketa said.

A tsunami crashed into the plant, north of Tokyo, on March 11, 2011, causing fuel-rod meltdowns at three reactors, radioactive contamination of air, sea and food and triggering the evacuation of 160,000 people.

It was the world's worst nuclear accident since Chernobyl in 1986.

On Wednesday, nuclear regulators said the latest leak at Fukushima represented a level-three "serious incident" on the UN's seven-point International Nuclear Event Scale (INES), raising the alert from level one, an "anomaly".

TEPCO has said puddles of water near the leaking tank were so toxic that anyone exposed to them would receive the same amount of radiation in an hour that a nuclear plant worker in Japan is allowed to receive in five years.

Source: Al Jazeera and agencies