Highly radioactive water from Japan's crippled Fukushima nuclear plant is pouring out at a rate of 300 tonnes a day, officials said on Wednesday, as Prime Minister Shinzo Abe ordered the government to step in and help in the clean-up.

The revelation amounted to an acknowledgement that plant operator Tokyo Electric Power Co (Tepco) has yet to come to grips with the scale of the catastrophe, 2 1/2 years after the plant was hit by a huge earthquake and tsunami. Tepco only recently admitted water had leaked at all.

Calling water containment at the Fukushima Daiichi station an "urgent issue," Abe ordered the government for the first time to get involved to help struggling Tepco handle the crisis.

The leak from the plant 220 km (130 miles) northeast of Tokyo is enough to fill an Olympic swimming pool in a week. The water is spilling into the Pacific Ocean, but it was not immediately clear how much of a threat it poses.

As early as January this year, Tepco found fish contaminated with high levels of radiation inside a port at the plant. Local fishermen and independent researchers had already suspected a leak of radioactive water, but Tepco denied the claims.

Tetsu Nozaki, the chairman of the Fukushima fisheries federation said he had only heard of the latest estimates of the magnitude of the seepage from media reports.

Environmental group Greenpeace said Tepco had "anxiously hid the leaks" and urged Japan to seek international expertise.

"Greenpeace calls for the Japanese authorities to do all in their power to solve this situation, and that includes increased transparancy...and getting international expertise in to help find solutions," Dr. Rianne Teule of Greenpeace International said in an emailed statement.

In the weeks after the disaster, the government allowed Tepco to dump tens of thousands of tonnes of contaminated water into the Pacific in an emergency move.

But the escalation of the crisis raises the risk of an even longer and more expensive clean-up, already forecast to take more than 40 years and cost $11 billion.

The admission further dents the credibility of Tepco, criticised for its failure to prepare for the tsunami and earthquake, for a confused response to the disaster and for covering up shortcomings.

"We think that the volume of water (leaking into the Pacific) is about 300 tonnes a day," said Yushi Yoneyama, an official with the Minister of Economy, Trade and Industry, which oversees energy policy.

Tatsuya Shinkawa, a director in METI's Nuclear Accident Response Office, told reporters the government believed water had been leaking for two years, but Yoneyama told Reuters it was unclear how long the water had been leaking at the current rate.

Shinkawa described the water as "highly" contaminated.

The water is from the area between the crippled reactors and the ocean, where Tepco has sought to block the flow of contaminated water by chemically hardening the soil.

Tetsu Nozaki, head of the Fukushima fisheries federation called for action to end the spillage.

"If the water was indeed leaking out at 300 tonnes a day for more than two years, the radiation readings should be far worse," Nozaki told Reuters. "Either way, we have asked Tepco to stop leaking contaminated water into the ocean."

Abe steps in

Abe ordered his government into action. The contaminated water was "an urgent issue to deal with", he told reporters after a meeting of a government task force on the disaster.

"Rather than relying on Tokyo Electric, the government will take measures," he said after instructing METI Minister Toshimitsu Motegi to ensure Tepco takes appropriate action.

The prime minister stopped short of pledging funds to address the issue, but the ministry has requested a budget allocation, an official told Reuters.

The Nikkei newspaper said the funds would be used to freeze the soil to keep groundwater out of reactor buildings - a project estimated to cost up to 40 billion yen ($410 million).

Tepco's handling of the clean-up has complicated Japan's efforts to restart its 50 nuclear power plants. All but two remain shut since the disaster because of safety concerns.

That has made Japan dependent on expensive imported fuels.

An official from the newly created nuclear watchdog told Reuters on Monday that the highly radioactive water seeping into the ocean from Fukushima was creating an "emergency" that Tepco was not containing on its own.

Abe on Wednesday asked the regulator's head to "do his best to find out the cause and come up with effective measures".

Tepco pumps out some 400 tonnes a day of groundwater flowing from the hills above the nuclear plant into the basements of the destroyed buildings, which mixes with highly irradiated water used to cool the fuel that melted down in three reactors.

Tepco is trying to prevent groundwater from reaching the plant by building a "bypass", but recent spikes of radioactive elements in sea water prompted the utility to reverse denials and acknowledge that tainted water is reaching the sea.

Tepco and the industry ministry have been working since May on a proposal to freeze the soil to prevent groundwater from leaking into the reactor buildings.

Similar technology is used in subway construction, but Chief Cabinet Secretary Yoshihide Suga said that the vast scale of Tepco's attempt was "unprecedented in the world."

The technology was proposed by Kajima Corp, , a construction company already heavily involved in the clean-up.

Experts say maintaining the ground temperatures for months or years would be costly. The plan is to freeze a 1.4 km (nearly one mile) perimeter around the four damaged reactors by drilling shafts into the ground and pumping coolant through them.

"Right now there are no details (of the project yet). There's no blueprint, no nothing yet, so there's no way we can scrutinise it," said Shinji Kinjo, head of the task force set up by the nuclear regulator to deal with the water issue.