Heavy rain at the Fukushima nuclear plant has caused a leak of radioactive water containing a cancer-causing isotope, possibly into the sea, its operator said on 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 Co and the government limiting the level of radioactive contamination in water leaving the plant.

Fukushima Governor Yuhei Sato (orange helmet) inspects the contaminated water tanks at TEPCO Fukushima nuclear power plant on Oct 15. JiJi Press via Agence France-Presse

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.

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

TEPCO has poured thousands of tons of water onto badly damaged reactors at Fukushima to keep them cool and prevent further 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.

Meanwhile, radiation cleanup in some of the most contaminated towns around Fukushima's nuclear power plant is behind schedule, so some residents will have to wait a few more years before returning, Japanese officials said on Monday.

Environment Ministry officials said they are revising the cleanup schedule for six of 11 municipalities in an exclusion zone from which residents were evacuated after three reactors at the Fukushima nuclear power plant went into meltdown following the March 2011 earthquake and tsunami. The original plan called for completing all decontamination by next March.

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 toward Japan. Forecasters expect it will hit later in the week.

In August, 300 tons 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, whose experts have been in Japan, sounded a largely positive note on progress in dealing with the mess in and around the Fukushima plant.

AFP-AP