About 12 tonnes of radioactive water has leaked at Japan's crippled Fukushima Daiichi nuclear power plant, with the facility's operator saying Thursday that some may have flowed into the Pacific Ocean.

Tokyo Electric Power Co. said the leak was found early Thursday from a pipe attached to a temporary decontamination system, and the water had already gone through some of the cleansing process.

The water, once it has been used to cool the reactors, contains massive amounts of radioactive substances and is put into the water-processing facility so it can be recycled for use as a coolant.

"Our officials confirmed that cooling water leaked at a joint in the pipes," a TEPCO spokesman told AFP, adding that "it is possible that part of the water may have flowed outside the facility and poured into the ocean".

The leak has since been plugged, the spokesman added, saying the utility was probing the cause of the accident and how much, if any, water flowed into the Pacific.

The accident was the latest of several leaks of radioactive water at the troubled plant, undermining the government's claim made in December that the shuttered Fukushima reactors were now under control.

In one incident last month, about 120 tonnes of radioactive water leaked at the plant's water decontamination system and about 80 litres (21 gallons) seeped into the ocean, according to TEPCO.

The plant about 220 kilometres (135 miles) northeast of Tokyo was crippled by meltdowns and explosions caused by Japan's massive earthquake and tsunami in March last year.

Radiation was scattered over a large area and made its way into the sea, air and food chain in the weeks and months after the disaster.

Tens of thousands of people were evacuated from their homes around the plant and swathes of this zone remain badly polluted. The clean-up is proceeding slowly, amid warnings that some towns could be uninhabitable for three decades.