Experts from the International Atomic Energy Agency urged Japan Friday to ensure decontamination efforts in the wake of the Fukushima crisis stay focused on reducing radiation doses to the public.

The group of 12 international experts released a preliminary report Friday praising the commitment of Japanese authorities and residents to clean-up operations, but also urged efficiency as it concluded a fact-finding mission.

The report warned against overly "conservative" efforts on decontamination in areas such as forests that did not pose a particularly high exposure threat to the population, which would also result in generating more waste material.

"Japan has to deal with a very huge problem," Juan Carlos Lentijo, head of radiation protection at the Spanish nuclear regulatory authority, said to Goshi Hosono, state minister in charge of the nuclear accident.

A towering tsunami released by the March 11 earthquake crippled cooling systems and triggered reactor meltdowns at the Fukushima Daiichi plant, leading to the release of radiation over wide areas of northeast Japan.

Tens of thousands of people remain evacuated from areas in and beyond the 20-kilometre no-go zone around the Fukushima plant. Fully restoring those areas is expected to take decades.

The task of restoring towns and villages even in lightly contaminated zones is complicated, with high costs and logistical issues of where to store soil contaminated with radioactive caesium after it is removed.

However, Lentijo said: "We want to explain there is a need for looking for a more realistic approach."

Japan "is advised to consider explaining to the public the importance of focusing on radiation doses that may actually be received by people, rather than on data containing contamination levels," the report said.

"The investment of time and effort in removing contamination beyond certain levels from everywhere, such as all forest areas where the additional exposure is relatively low, does not automatically lead to reduction of doses for the public," the report said.

"It also involves a risk of generating unnecessarily huge amounts of residual material."

A final report will be provided to the Japanese government by November 15 this year, the team said.

Over a nine-day trip the experts visited the crippled nuclear plant and many other locations in Fukushima prefecture, including the cities of Minamisoma and Date, in addition to Iitate village where local residents have been evacuated.

Seven months on from the disaster, radiation emissions from the plant have been reduced as crews work to put the facility into a stable state of cold shutdown by January.

In September Japan eased evacuation advisories for five areas near the crippled Fukushima nuclear plant as it looks to convince nearly 30,000 residents that it is safe to return home.