Heavy rain at the Fukushima nuclear plant caused a leak of radioactive water containing a cancer-causing isotope, possibly into the sea, its operator said Monday, as a typhoon approaching Japan threatened further downpours.

The leak is the latest in a long line of setbacks at the site, and further undermines agreements between operator Tokyo Electric Power (TEPCO) and the government which limit the level of radioactive contamination in water that goes outside the plant.

TEPCO said a barrier intended to contain radioactive overflow was breached in one spot by water contaminated with strontium-90 at 70 times the legal limit for safe disposal.

Strontium-90 is produced during nuclear reactions. It accumulates in bones and remains potent for many years, and causes several types of cancer in humans.

The admission came as a team of experts from the UN's nuclear watchdog ended their review of Japan's progress in cleaning up the wider environment after the tsunami-sparked meltdowns of March 2011 created the worst atomic disaster in a generation.

TEPCO has poured thousands of tonnes of water onto badly-damaged reactors at Fukushima to keep them cool and prevent repeat meltdowns.

This huge volume of water must be stored in large tanks until it is cleaned of the radioactive substances it picks up in the cooling process.

Rain worsens the problem because as it hits polluted surfaces, it becomes contaminated, meaning TEPCO needs to scoop it all up for storage and treatment.

While the storage tanks all appeared to have survived the battering from heavy rain on Sunday, the concrete overflow barriers around them were not high enough to contain the rainwater runoff in several places.

Meteorologists say a typhoon that is likely to bring further heavy rain is churning its way slowly towards Japan. Forecasters expect it will hit later in the week.

In August, 300 tonnes of badly-polluted water leaked from a tank. It is now believed to have mixed with groundwater that is on its way to the sea.

Meanwhile, the International Atomic Energy Agency (IAEA), whose experts have been in Japan, sounded a largely positive note on progress in dealing with the mess around the Fukushima plant.

The mission, which came at Tokyo's request, is a follow-up to an IAEA visit last year.

Japan must review its communication strategies to improve public confidence in the process, the team said in its interim report.

"Japan has done an enormous amount to reduce people's radiation exposure in the affected areas, to work towards enabling evacuees to go back to their homes and to support local communities in overcoming economic and social disruption," team leader Juan Carlos Lentijo said in a prepared statement.

The report said that "food safety measures (have) protected consumers and improved consumer confidence in farm produce".

Existing screening programmes ensure that whatever is shipped to the market place "is safe for consumers," he told a press conference.

"We ate fish" during the mission in Fukushima, he told reporters.

The Japanese government must better inform the public to reduce uncertainty as well as explain the costs and benefits of public programmes, Lentijo said.

But he hailed what Japan has done so far, at least outside of the Fukushima plant.

Despite official endorsements like this, the Japanese public and international consumers continue to be sceptical about the safety of food from the area.

Prime Minister Shinzo Abe, who has repeatedly insisted the situation is under control, has said he eats Fukushima-grown rice daily.