The UN atomic watchdog said Wednesday radiation in a village outside the evacuation zone around a stricken Japanese nuclear plant was above safe levels, urging that Japan reassess the situation.

In its first such call, the International Atomic Energy Agency added its voice to that of Greenpeace in warning over radioactivity in Iitate village, where the government has already told residents not to drink tap water.

Japan has struggled to contain its nuclear emergency since a 14-metre (45-foot) tsunami hit the Fukushima plant after a huge quake on March 11, with radioactive substances entering the air, sea and foodstuffs from the region.

Iitate village is 40 kilometres (25 miles) northwest of the crisis-hit plant -- outside both the government-imposed 20 kilometre exclusion zone and the 30-kilometre "stay indoors" zone.

"The first assessment indicates that one of the IAEA operational criteria for evacuation is exceeded in Iitate village," the IAEA's head of nuclear safety and security, Denis Flory, told reporters in Vienna on Wednesday.

The watchdog advised Japanese authorities to "carefully assess the situation and they have indicated that it is already under assessment," Flory said.

But he added the IAEA, which does not have the mandate to order national authorities to act, was not calling for a general widening of the exclusion zone.

Amid public fears over contamination from the world's worst nuclear crisis since Chernobyl, campaign group Greenpeace called earlier Wednesday for the zone to be expanded to evacuate everyone within 30 kilometres of the plant.

It said the government should consider moving children and pregnant women beyond that, after urging Tuesday that residents of Iitate be moved.

Radiation expert Jan van der Putte said "remaining in Iitate for just a few days could mean receiving the maximum permissible annual dose of radiation".

On Wednesday he added: "Exposing a large number of people to this level of radiation creates a collective risk which is very significant over a long term, in terms of years. Our main concern is an increased incidence of cancer."

The reading in Iitate village was 2 megabecquerels per square metre, a "ratio about two times higher than levels" at which the IAEA recommends evacuations, said the head of its Incident and Emergency Centre, Elena Buglova.

The government on Monday told residents of Iitate not to drink tap water, with media reports saying 4,000 residents would be given bottled water.

Radiation worries in the area worsened Wednesday when iodine-131 detected in the Pacific Ocean water near Fukushima surged to a new high of 3,355 times the legal limit, officials said, against a previous high of 1,850 times the limit.

"The figures are rising further," said nuclear safety agency spokesman Hidehiko Nishiyama. "We need to find out as quickly as possible the cause and stop them from rising any higher."

Japan has halted vegetable and dairy shipments from four prefectures around the plant and briefly said tap water in Tokyo should not be drunk by infants, but called for calm and said it was taking these measures as a precaution.

However pressure to come up with fresh ideas intensified Wednesday, as Tokyo Electric Power Company (TEPCO), which operates the stricken plant, admitted it had no idea when the situation would be under control.

"Key factors are still unknown, such as how the nuclear incident will come to an end... In a word, the very difficult situation is expected to continue," TEPCO chairman Tsunehisa Katsumata told reporters.

With crucial control room functions still disabled, experts are not sure what exactly is happening inside the stricken reactors -- and some international experts warned that a meltdown may already be in progress.

One of them is Richard Lahey, who was head of safety research for boiling-water reactors at General Electric when it installed the Fukushima units, and who was quoted by Britain's Guardian newspaper.

Available reactor and radiation data from the troubled unit two "suggest that the core has melted through the bottom of the pressure vessel" and onto the concrete floor, he was quoted as saying by the daily.

"I hope I am wrong, but that is certainly what the evidence is pointing towards."

Japan faces a dilemma in containing the crisis: it must pump water into reactors to stop them from overheating, even as highly radioactive runoff leaks out, halting crucial repair work and threatening the environment.

On Wednesday it was weighing a series of solutions, from draping reactors with special fabric to sending in military robots to do the risky work.

One stop-gap measure reported by local media involved covering three badly damaged outer reactor buildings with special fabric caps and fitting air filters to limit radiation.

Another plan was to anchor an empty tanker off reactor two, so that workers can pump several Olympic swimming pools' worth of highly-radioactive runoff water into its hull, media said.

"We are in an unprecedented situation, so we need to think about different strategies, beyond what we normally think about," an official with the nuclear safety agency told AFP.

Chief Cabinet Secretary Yukio Edano said the government and nuclear experts were discussing "every possibility, including those mentioned in the press".

Workers will on Thursday begin to carpet two-thirds of the plant's 1.2 hectares (three acres) of grounds with a resin to trap the radioactive particles, the agency said later.

The United States has lent Japan robots of a model battle-tested in Iraq and Afghanistan that can navigate, film and clear rubble in the blast-hit reactor buildings, which humans cannot enter because of very high radiation levels.

US President Barack Obama vowed continued help as he talked to Japanese Prime Minister Naoto Kan for the third time since the disaster, which has left over 11,000 confirmed dead and more than 16,000 listed as missing.

"The United States is determined to support the people of Japan in their efforts to deal with the devastating effects of this tragedy, both in the short and the long term," Obama said, according to the White House.

The strain of the crisis appeared to have taken a toll on TEPCO's president Masataka Shimizu, 66, who was hospitalised Tuesday evening with high blood pressure and dizziness, having not appeared in public for over two weeks.

The company's shares have plunged to about a fifth of pre-quake levels amid heavy criticism, most recently over news that it ignored expert warnings on the threat of a tsunami before a giant wave crashed into the plant on March 11.

TEPCO chairman Katsumata said he saw little chance that the four stricken reactors in the six-reactor complex could ever resume operations.