Environmental group Greenpeace has been monitoring levels of radioactivity at the village of IitateUN nuclear monitors have advised Japan to consider expanding the evacuation zone around the stricken reactors at the Fukushima Daiichi plant.
An exclusion zone with a radius of 20km (12 miles) is currently in place but the UN says safe radiation limits have been exceeded 40km away.
Meanwhile, radioactive iodine levels in seawater near the plant reached a new record - 4,385 times the legal limit.
It was the highest reading since the quake which hit the plant on 11 March.
Radiation may be leaking from the damaged plant continuously, the country's nuclear and industrial safety agency (Nisa) said.
The plant's operator, Tokyo Electric Power (Tepco), announced on Wednesday that the four stricken reactors would be decommissioned.
A massive tsunami which resulted from the quake is now known to have claimed at least 11,417 lives, with 16,273 people still reported missing by police, three weeks on.
'No immediate action'The UN's nuclear watchdog (International Atomic Energy Agency, IAEA) found safe radiation limits had been exceeded at the village of Iitate, 40km north-west of the nuclear plant.
"The highest values were found in a relatively small area in the north-west from the Fukushima power plant and the first assessment indicates that one of the IAEA operational criteria for evacuation is exceeded in Iitate village," senior IAEA official Denis Flory said.
Japan's top government spokesman, Chief Cabinet Secretary Yukio Edano, said on Thursday that the IAEA had advised the government to "carefully assess the situation on the basis of this report".
"I don't think that this is something of a nature which immediately requires such action," he told reporters.
"But the fact that the level of radiation is high in the soil is inevitably pointing to the possibility that the accumulation over the long term may affect human health.
"Therefore, we will continue monitoring the level of radiation with heightened vigilance and we intend to take action if necessary."
The US and UK earlier advised their citizens in Japan to keep at least 80km from the plant.
The US is to send a 140-member radiation control team to assist Japanese authorities at the plant, Kyodo News agency quoted Japan's military chief, General Ryoichi Oriki, as saying.
Seawater readingsThe new radioactive iodine levels in seawater near the plant - 4,385 times the legal limit - were reported by Nisa.
On Wednesday, radioactive iodine was estimated to be 3,355 times the legal limit, while previously the figure had been put at 1,850 times the legal limit.
Radioactive iodine was blamed for the high incidence of thyroid cancer among children exposed to fallout from the Chernobyl nuclear disaster in 1986.
Nisa's director-general, Hidehiko Nishiyama, told reporters the new levels did not present a health risk because nearby residents had already been evacuated.
Officials point out that no fishing is allowed in the area. Because the radiation should disperse as it is carried away by the tides, they do not think there will be a serious threat to marine life.
