Officials from the UN's International Atomic Energy Agency say higher-than-recommended levels of radiation have been detected outside the evacuation zone of the Japan nuclear disaster.

The officials emphasized that the readings, at Iitate village, about 40 kilometres from the Fukushima Dai-ichi nuclear complex, were sporadic and were registered only at one measuring point.

"The first assessment indicates that one of the IAEA operational criteria for evacuation is exceeded in Iitate village," IAEA official Denis Flory told a news conference.

Elena Buglova, another IAEA official, says the reading was 2 megabecquerels per square metre. She said "as a ratio, it was about two times higher" than levels at which the agency recommends evacuations.

Residents within 20 kilometres of the plant were evacuated two weeks ago, while those up to 30 kilometres have been urged to leave voluntarily or to stay indoors.

However, the IAEA stopped short of officially calling for an evacuation for the village of Iitate. Instead, they said that Japan should "carefully assess the situation."

Earlier in the day, Japan's nuclear safety agency announced it had detected the highest levels yet of radiation in nearby seawater.

The state said the readings showed a spike in radioactive iodine in the seawaters near the plant that are at 3,355 times the legal limit. But the agency played down the worry, saying people had already left the area and fishing had stopped.

Experts noted that radioactive iodine is short-lived, with a half-life of just eight days, and does not tend to accumulate in shellfish. They also noted that the vastness of the Pacific Ocean should dilute the radiation, limiting the danger.

Still, for an island nation where fish is central to the diet, the news is hardly reassuring to most.

It's now been nearly three weeks since the March 11 tsunami engulfed the Fukushima plant, and Tokyo Electric Power Co. (TEPCO) is still struggling to bring the radiation crisis under control.

It's already been determined that radiation has seeped into the soil and seawater and made its way into produce, raw milk and even tap water as far as Tokyo, 220 kilometres to the south.

Concern over contamination also continued to spread abroad, as low levels of radiation were found in a sample of milk in the U.S. state of Washington.

However, U.S. officials said the levels would drop relatively quickly in the coming days and that the levels of Iodine-131 were 5,000 times below regulation.

Milk radiation readings

The U.S. and other countries had already stopped importing Japanese milk products from the affected area, and officials said that the readings should not be alarming.

"Radiation is all around us in our daily lives, and these findings are a miniscule amount compared to what people experience every day," said Patricia Hansen, a senior scientist from the Food and Drug Administration.

"A person would be exposed to low levels of radiation on a round-trip cross-country flight, watching television, and even from construction materials."

Other food products, including seafood, from Japan will continue to be sold in the U.S. However, the products will be screened for radiation first.

Also Wednesday, TEPCO conceded that it would have to decommission four of the six reactors at the Fukushima facility. The seawater used to cool the reactors and spent fuel pools would have been corrosive to the reactors, leaving them inoperable.

More consultation would be needed to determine the fates of reactors 5 and 6, which were not operating at the time of the quake and tsunami and were cooled down safely.

Chief Cabinet Secretary Yukio Edano was unable to say when the country might feel assured that the problem at Fukushima was contained.

"We are not in a situation where we can say we will have this under control by a certain period," Edano told a Wednesday briefing.

The stress of the disaster has taken its toll on TEPCO President Masataka Shimizu, who was sent to a hospital late Tuesday.

Shimizu, 66, has not been seen in public since two days after the disaster hit, when he attended a March 13 news conference in Tokyo.

His absence since has raised speculation that he had suffered a breakdown. For days, officials deflected questions about Shimizu's whereabouts, saying he was "resting" at company headquarters.

Spokesman Naoki Tsunoda said Wednesday that Shimizu had been admitted to a Tokyo hospital after suffering dizziness and high blood pressure.

In an effort to offer help and support, France is flying in two experts from its state-owned nuclear reactor maker Areva and its CEA nuclear research body.

France produces about 75 per cent of its power from reactors so it is considered a global leader in the industry.

French President Nicolas Sarkozy also plans to visit Tokyo on Thursday, becoming the first foreign leader in Japan since the disaster.