Japanese officials spearheading ongoing clean-up efforts around the crippled Fukushima nuclear plant are being urged to take a more aggressive approach.

A preliminary report from the United Nation's nuclear watchdog agency released Friday said Japan must avoid becoming too “conservative” in its clean-up efforts. The International Atomic Energy Agency report also encourages Japan to focus decontamination efforts on measures that do the most to limit the public's exposure to radiation.

Despite the concerns about Japan's approach, the IAEA team praised officials for their commitment to the clean-up effort.

The 12-member team spent nine days in Japan looking at recovery efforts and is expected to issue its final report next month.

Tens of thousands of people remain evacuated from areas in and around a 20-kilometer zone around the the disabled Fukushima-Daiichi plant. The power plant has been leaking radiation since the March 11 earthquake and tsunami devastated much of coastal northeastern Japan, leaving 20,000 people dead or missing.

Japanese officials said earlier this week that new radiation threats have been discovered in Tokyo and the nearby city of Yokohama, suggesting that leaked radiation from Fukushima remains a problem throughout much of Japan.

They said radiation in a residential area in Tokyo, about 235 kilometers southwest of Fukushima, was measured at levels higher than in some parts of the evacuation zone surrounding the Fukushima plant.

Also this week, Japanese doctors began long-term testing on 360,000 children to learn the extent to which radiation may have exposed them to an increased risk of thyroid cancer.

No one has yet died from radiation exposure. But the screenings were launched after a recent survey showed that at least 10 of 130 children evacuated from Fukushima prefecture had thyroid abnormalities. No direct link was established between the medical conditions and the leaking radiation.

Doctors plan to first examine 5,000 children who were living closest to the plant when meltdowns occurred at three of the plant's six nuclear reactors.