Puddles with extremely high radiation levels have been found near water storage tanks at Japan's crippled Fukushima nuclear plant, as it moved to contain the latest of several toxic water threats, AFP reports citing the operator.
The radiation level, measured around 50 centimetres (20 inches) above the toxic water surface, was about 100 millisieverts per hour, Tokyo Electric Power (TEPCO) said.
Scientists consider that level of radiation as harmful to human health.
An estimated 300 tonnes of water is believed to have leaked out from a huge radioactive water storage tank at the vast site, with the leak believed to be continuing Tuesday, TEPCO and the Nuclear Regulation Authority (NRA) said.
TEPCO denied that toxic water had flowed into the adjacent Pacific ocean, but the regulator ordered the utility to study the possibility that it had escaped through nearby drains.
The agency released a preliminary assessment that the situation amounted to a level one incident on an eight-point international scale from zero to seven -- seven being the most serious reading.
"We have instructed TEPCO to find the source of contaminated water -- from which tank the water is leaking -- and to seal the leakage point," a regulatory official told AFP.
"We have also instructed them to retrieve contaminated soil to avoid a further expansion of toxic water, and to strengthen monitoring of the surrounding environment."
There were no significant changes in radiation levels outside the plant, he added.
Drain valves on the water tank may have been left open, allowing the water to flow out, TEPCO said.
"At 7:00 pm (on Monday, 1000 GMT), we started retrieving the escaped water," the company said.
An employee at the utility found water leaking from a valve at about 9:50 am local time Monday, it added.
TEPCO has faced a growing catalogue of incidents at the plant including several leaks of radioactive water, more than two years after the worst nuclear disaster in a generation triggered by a huge quake and tsunami in March 2011.
The company -- which faces huge clean-up and compensation costs -- has struggled with a massive amount of radioactive water accumulating as a result of continuing water injections to cool reactors.
The embattled utility in July admitted for the first time that radioactive groundwater had been leaking outside the plant and this month started pumping it out to reduce leakage into the Pacific.
The problems have led the Japanese government and its nuclear regulator to say they would get more directly involved in the cleanup at Fukushima plant, which had largely been left in the hands of the company.
While no one is officially recorded as having died as a direct result of the meltdowns at Fukushima's reactors, large areas around the plant had to be evacuated, with tens of thousands of people still unable to return to their homes.