Japan's prime minister Wednesday said Tokyo would get more involved in cleaning up the crippled Fukushima nuclear plant, as he described as "urgent" a battle to stop radioactive water from leaking into the ocean.

The government's more prominent role comes as critics attack plant operator Tokyo Electric Power and its handling of the more than two-year-old atomic crisis, the worst nuclear accident in a generation.

The embattled utility -- kept afloat by a government bail out -- last month admitted for the first time that radioactive groundwater had been leaking outside the plant, confirming long-held suspicions of ocean contamination from its shattered reactors.

It has since said tainted water has been escaping into the Pacific for more than two years.

On Wednesday, an official at Japan's industry ministry said Tokyo estimates a whopping 300 tonnes of contaminated water from a newly discovered leak site may be seeping into the ocean daily.

"But we're not certain if the water is highly contaminated," he added.

A French expert said the environmental risk posed by the leaks was small compared to the overall radioactive contamination from the disaster.

"We are not seeing anything new in our measurements of the ocean water, sediment or fish. I think it is negligible," said Jerome Joly, deputy director general of the French Institute for Radiological Protection and Nuclear Safety, IRSN, which has closely monitored the Fukushima disaster.

"Japan, in this geographical area, benefits from two currents travelling along the coast eastwards to the Pacific, and they play a valuable dilution role," he told AFP.

The leaks however have triggered fresh worries over the plant's precarious state and TEPCO's ability to deal with a growing list of problems after its reactors were swamped by a tsunami in March 2011, sending them into meltdown.

The company has also faced widespread criticism over its lack of transparency in making critical information public since the disaster.

On Wednesday, Prime Minister Shinzo Abe said his government would beef up efforts to help with the expected decades-long clean up, which has largely been left to TEPCO to handle.

"Stabilising the Fukushima plant is our challenge," Abe said at a meeting of the government's disaster task force.

"In particular, the contaminated water is an urgent issue which has generated a great deal of public attention."

His Liberal Democratic Party wants to restart the country's reactors, which were switched off in the wake of the crisis, if their safety can be assured.

Abe said the clean-up would no longer be left to TEPCO alone. He also called for "swift and steady measures" on the toxic water issue.

Tokyo would now help foot the bill, Abe said, the first time that it has committed extra funds to deal with the growing problem.

The vast utility is already facing billions of dollars in clean-up and compensation costs over the accident.

TEPCO had previously reported rising levels of cancer-causing materials in groundwater samples at Fukushima. But until last month, the company had insisted it had halted toxic water from leaking beyond its borders.

In May, Tokyo ordered the company to build new barriers to contain the massive amounts of water which are used to keep the reactors cool, a measure that could cost up to 40 billion yen ($410 million).

There are growing fears that existing safeguards would soon be overwhelmed, as TEPCO scrambles to find ways to store the water.

"The worsening leaks of contaminated water at the Fukushima nuclear plant prove TEPCO is incapable of dealing with the disaster," Greenpeace said in a statement on Tuesday.

"Japan's authorities must now step in and ensure action is finally taken to stop the leaks," it added.

The country's Nuclear Regulation Authority (NRA) has said it plans to pull together two dedicated teams to probe water contamination and its impact on the ocean's ecosystem.

More than 18,000 people died when the tsunami slammed into Japan's northeast coast on March 11, 2011.

While no one is officially recorded as having died as a direct result of the meltdowns at Fukushima, large areas around the plant had to be evacuated with tens of thousands of people still unable to return to their homes. bur-mlr/mtp/bm