The operator of the crippled Fukushima nuclear plant said Thursday it had found new radiation hotspots near tanks storing radioactive water, but no new leaks.

Around 300 tonnes of toxic liquid is believed to have escaped from one of the tanks that hold polluted water, some of which was used to cool the broken reactors, in an episode dubbed the most serious in nearly two years.

Plant owner Tokyo Electric Power Co (TEPCO) warned some of it might have flowed into the Pacific Ocean.

And during inspections Thursday of around 300 other tanks of a similar design, TEPCO said while no more holes had been found, two areas were a cause for concern.

"We have confirmed two spots where radiation doses are high" near two other tanks, a company statement said.

But the levels of water in these two tanks have not changed since they were pressed into service to store contaminated water and the ground around them was dry, it added.

The inspections were prompted by the discovery of a leak that the company said may have carried radioactive materials out to sea, with the country's nuclear watchdog voicing concerns that there could be similar leaks from other containers.

On Wednesday, nuclear regulators said the leak 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".

The quake and tsunami-sparked meltdowns at the plant in March 2011 were ultimately categorised as level seven on the INES scale. The Chernobyl disaster in 1986 is the only other incident to have been given the most serious ranking.

TEPCO has said puddles of water near the holed 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.

The absence of a volume gauge on the 1,000-tonne tank made detecting the problem more difficult, experts say.

Scientists say levels of radiation in the ocean in several spots along the Fukushima coast had been recovering.

"It is too early to estimate the impact of the latest leak," said Masashi Kusakabe, researcher at Marine Ecology research Institute.

"All we can do is to continue monitoring levels of marine radiation very carefully," Kusakabe said.

Jota Kanda, an oceanographer and professor at Tokyo University of Marine Science and Technology, said: "It is inevitable that some water has reached the sea. So far its impact on the marine environment is limited, but it will be a different story if more leaks happen."

TEPCO -- which faces huge clean-up and compensation costs -- has struggled to cope with the after-effects of the disaster.

More than two years after the meltdowns, it continues to be beset by difficulties, chief among which is how it should handle the vast amounts of water used to cool the broken reactors.

Around 1,000 tanks of varying sizes have been installed at the site to contain it, but experts warn this can only be a temporary fix.

A series of problems, and delays in announcing them to the public, have added to the impression that the huge utility is not on top of the clean-up.

TEPCO in July admitted for the first time that radioactive groundwater had been leaking outside the plant.

This month it started pumping it out to reduce leakage into the Pacific and this week said 30 trillion becquerels of strontium and caesium, possibly cancer-causing substances, could have entered the ocean since May 2011 from this leak.

While no one is officially recorded as having died as a direct result of the radiation released by the meltdowns, large areas around the plant had to be evacuated.

Tens of thousands of people are still unable to return to their homes, with scientists warning some areas may have to be abandoned.

On Thursday, fishermen based in the north of Fukushima decided to suspend 15-month-old trial operations because of pollution fears, Kyodo news agency said.