Another suspected leak of radioactive water was discovered at Japan's crippled Fukushima Daiichi nuclear power station on Tuesday, its operator said, the latest in a series of problems at the plant.

Contaminated water may have leaked into the ground from an underground reservoir at the tsunami-damaged facility, following similar leaks at two others during the weekend, Tokyo Electric Power Co (TEPCO) said.

TEPCO insisted that the radioactive water was unlikely to flow into the sea.

The reservoir, which is enclosed, stores water that has been used to cool down the reactors after caesium is removed but while other radioactive substances remain.

The latest leak fear was reported at the No. 1 pool, the destination for radioactive water being transferred from the flawed No. 2 facility, a TEPCO spokesman said, adding the transfer had now been suspended.

The leaks came after two power failures on the systems which keep spent atomic fuel cool at the plant. The first, which knocked out some equipment for nearly 30 hours, was caused by a rat that had got into the electrics.

The second failure, which resulted in a suspension of cooling for just a few hours, was the result of an error while work was being carried out to rodent-proof the electrics, the company has said.

Nuclear fuel, even after use, must be kept cool to prevent it from overheating and beginning a self-sustaining atomic reaction that could lead to meltdown.

Failures at the plant, more than two years after it was hit by the tsunami of March 2011 that caused meltdowns and sent tens of thousands fleeing from their homes, have underlined the precarious state of the facility.

Next week an International Atomic Energy Agency mission will visit the plant to monitor the process of decommissioning and check problems including the recent water leaks, a government official said.

The 12-member team, the third of its kind since the original disaster, is scheduled to release a preliminary report on April 22, the official added.

Economy, Trade and Industry Minister Toshimitsu Motegi said it was up to TEPCO to resolve the leak problems, but "we think fundamental measures are necessary".