Radiation measured at a village 40km from Japan's crippled nuclear plant is falling by the day, the UN atomic agency said today, two days after warning the level exceeded a criterion for evacuation.

Wednesday's statement by the International Atomic Energy Agency (IAEA) had added to pressure on Japan's government to extend the exclusion zone beyond 20km around the severely damaged Fukushima atomic power station.

But while the amounts of radioactive iodine particles detected in the soil at Iitate village appeared to be declining from high levels, the Vienna-based UN body said the overall situation at Fukushima remained very serious.

IAEA director general Yukiya Amano warned it would "take more time than people think" to end the crisis and stabilise the plant, which has leaked radioactivity since it was hit by a huge earthquake and tsunami on March 11th.

"Putting an end to the crisis will take some time, to stabilise the reactors will take more time," he told a news conference in Nairobi.

"I would say it will take more time than people think. It will not happen tomorrow or the day after tomorrow."

On Wednesday, the IAEA said radiation measured in Iitate village northwest of the power plant had exceeded recommended levels and urged Japanese authorities to "carefully assess" the situation there.

Japan's nuclear safety body had earlier rebuffed a call by environmental group Greenpeace to widen the evacuation zone, a move that could force tens of thousands of more people to leave their homes.

But today IAEA officials said further soil samples from Iitate showed the average value of radioactive iodine-131 down at 7 megabecquerels a square metre against 20 megabecquerels earlier -- twice the level of an IAEA criterion for evacuation.

"This value is lower than what was reported on Wednesday," senior agency official Gerhard Proehl told a news conference.

"Because there are more samples ... and together with the radioactive decay the situation improves daily," he said, adding 15 soil samples had been taken between March 19th and March 29th.

Japanese opposition politicians have lambasted prime minister Naoto Kan for sticking with the original exclusion area, nearly three weeks after an earthquake and tsunami sparked the world's worst nuclear crisis since Chernobyl in 1986.

But Denis Flory, an IAEA deputy director general, said the UN agency had never recommended evacuation from the area, saying this was up to Japan.

"It is in the hands of the Japanese government. They are assessing the situation and it is their role to take actions based on their assessment," Mr Flory said.

The disaster has prompted a rethink of nuclear power around the world, just as the technology was starting to regain momentum as a way to fight global warming.

"This will affect, of course, the future of use of nuclear power," Mr Amano said in Nairobi.

"The IAEA is not influencing member states to take decisions, but if countries opt for use of nuclear power, the IAEA is in a position to help to do it safely, securely and sustainably," he said.

Reuters