Pressure is mounting on Japan to expand the evacuation zone around its stricken nuclear power plant.
Officials say radiation may be flowing continuously into the nearby sea, where contamination is now 4000 times the legal limit, CNN reports.
The United Nations nuclear watchdog, the International Atomic Energy Agency (IAEA), and Japan's own nuclear safety agency say the government should consider widening the 20-kilometre zone after high radiation was detected at twice that distance from the Fukushima facility.
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 United States and Britain earlier advised their citizens in Japan to keep at least 80 kilometres from the plant, the BBC reports.
The plant's operator, Tokyo Electric Power (Tepco), announced on Wednesday that the four stricken reactors would be decommissioned.
The plant was seriously damaged in the massive earthquake and tsunami that struck northern Japan on 11 March, killing at least 11,400 people. Police say 16,273 people are still reported missing.