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Japan's nuclear watchdog said Wednesday the crippled Fukushima reactors are very likely leaking highly radioactive substances into the Pacific Ocean.

Members of the Nuclear Regulation Authority voiced frustration at Tokyo Electric Power (TEPCO), which has failed to identify the source and the cause of spiking readings of radioactive materials in groundwater.

"It is strongly suspected that highly concentrated contaminated waste water has leaked to the ground and has spread to the sea," the authority said in its written review of TEPCO's recent announcements.

The giant utility that services Tokyo and its surrounding regions has said groundwater samples taken at the battered Fukushima Daiichi plant on Tuesday showed levels of possibly cancer-causing caesium-134 were more than 110 times higher than they were on Friday.

TEPCO has failed to identify the exact reasons for the increased readings but has maintained that the toxic groundwater was likely contained at the current location, largely by concrete foundations and steel sheets.

The company has admitted in recent weeks that water and soil samples taken at the Fukushima plant are showing high readings for other potentially dangerous substances, including caesium-137, tritium and strontium-90.

"I see (TEPCO) has not been able to find the cause of these spikes in readings," NRA Chairman Shunichi Tanaka told a meeting of its commissioners.

"TEPCO says there has not been meaningful impact on the environment. But we must see for sure what are the possibilities of this leading to oceanic contamination," said Toyoshi Fuketa, an NRA commissioner.

NRA officials are urging TEPCO to offer more detailed and credible data and make efforts to better explain to the public what it knows.

The substances were released by the meltdowns of reactors at the plant in the aftermath of the huge tsunami of March 2011, which swamped cooling systems at the plant.

Tens of thousands of people were forced from their homes by the threat of radiation in the planet's worst nuclear accident for a generation.

TEPCO is struggling to manage the clean-up at Fukushima, which scientists say could take up to four decades to complete.

Thousands of tonnes of water used to cool reactors is being stored on site, and technical set-backs -- including storage tank leaks and power outages -- occur frequently.

Critics of the utility say it adopts a head-in-the-sand approach to problems at the plant, where ad-hoc fixes have left equipment vulnerable to quakes and tsunami in tectonically volatile Japan.

Although the radiation leaks from the Fukushima accident are not officially recorded as having directly killed anyone, the earthquake and tsunami that caused them claimed more than 18,000 lives and was one of Japan's worst ever peacetime tragedies.