By Mayumi Negishi and Kazunori Takada
TOKYO (Reuters) - The United States became the first nation to block imports from ally Japan's radiation zone, saying it will halt milk, vegetable and fruit from areas near the tsunami-smashed nuclear plant because of contamination fears.
The Food and Drug Administration's decision to stop imports from four Japanese prefectures in the crisis-hit northeast crystallized international anxiety about the impact of the worst atomic crisis since Chernobyl in 1986.
Other nations may follow suit with formal bans. Some private importers have already stopped shipments from Japan anyway.
At the six-reactor Fukushima plant, crippled by a March 11 earthquake and tsunami, engineers are battling to cool reactors to contain further contamination and avert a meltdown .
Showing the widening problem, Japan said on Wednesday above-safety radiation levels had been discovered in 11 types of vegetables from the area, in addition to milk and water.
Officials still insisted, however, that there was no danger to humans and urged the world not to over react.
"We will explain to countries the facts and we hope they will take logical measures based on them," Japan's chief cabinet secretary Yukio Edano, who has been the government's public face during the disaster , told a news conference.
The Asian nation's worst crisis since World War Two may have caused $300 billion damage, sent shock waves through global financial markets, and left nearly 23,000 people dead or missing, mostly from flattened coastal towns.
More than a quarter of a million people are living in shelters, while rescuers and sniffer dogs comb debris and mud looking for corpses and personal mementoes .
Worsened by widespread ignorance of the technicalities of radiation, public concern is rising around the world and radioactive particles have been found as far away as Iceland.
Japan has already halted shipment of some food from the area and told people there to stop eating leafy vegetables.
Asian neighbors are inspecting imports for contamination, and Taiwan advised boats to stop fishing in Japanese waters.
Although there has been progress in restoring power to the Fukushima site 13 days after the accident, plant operator Tokyo Electric Power Co (TEPCO) said it needed more time before it could say the reactors were stabilized.
Technicians working inside an evacuation zone around the plant, 250 km (150 miles) north of Tokyo, have successfully attached power cables to all six reactors and started a pump at one to cool overheating nuclear fuel rods.
Concern is high over reactor No. 1 after its temperature rose to near 400 degrees Celsius, above a design limit of 302.
Ramping up pressure at the site, two workers were injured while restoring power, Kyodo news agency said.
And engineers at No. 2 reactor had to pull out when radiation hit 500 millisieverts per hour, in the danger zone.
As well as having its workers on the front line in highly dangerous circumstances, TEPCO is also facing accusations of a slow disaster response and questions over why it originally stored more uranium at the plant than it was designed to hold.
Vienna-based U.N. watchdog, the International Atomic Energy Agency (IAEA), expressed concern about a lack of information from Japanese authorities. It cited missing data on temperatures of spent fuel pools at the facility's reactors 1, 3 and 4.
"We continue to see radiation coming from the site ... and the question is where exactly is that coming from?" said a senior IAEA official, James Lyons.
U.S. Defense Secretary Robert Gates said he was concerned about radioactive fallout affecting the U.S. 55,000 troops in and around Japan, many involved in a massive relief operation for Washington's close ally. "We're also deeply concerned about the wellbeing of our Japanese allies," he said.
Experts said tiny traces of radioactive particles, measured by a network of monitoring stations as they spread eastwards from Japan across the Pacific, North America, the Atlantic and to Europe, were far too low to cause any harm to humans.
"It's only a matter of days before it disperses in the entire northern hemisphere," said Andreas Stohl, a senior scientist at the Norwegian Institute for Air Research
The Japan crisis has dealt a blow to the nuclear power industry around the world. Italy became the latest nation to re-assess its programme, announcing a one-year moratorium on site selection and building of plants
WORLD'S COSTLIEST DISASTER
Crisis in the world's third-biggest economy -- and its key position in global supply chains, especially for the auto and technology sectors -- has added to global market jitters, also affected by conflict in Libya and unrest in the Middle East.
Asian shares fell on Wednesday, with Tokyo's Nikkei shedding more than 1 percent as investors took profits from a two-session bounce. Japanese stocks are about 8 percent below their close on the day the 9.0 magnitude earthquake struck.
Toyota said it would delay the launch in Japan of two additions to the Prius line-up, a wagon and a minivan, from the originally planned end-April due to production disruptions.
The tsunami and earthquake are the world's costliest ever natural disaster , with the government estimating damage at 15-25 trillion yen ($185 billion-$308 billion), the Nikkei newspaper said.
The upper end of that range would equate to about 6 percent of Japan's gross domestic product (GDP).
The official death toll has risen to 9,199, but with 13,786 people still reported missing, it is certain to rise.
There are reports dozens of survivors, mostly elderly, have died in hospitals and evacuation centers due to a lack of proper treatment, or simply because of the cold. It is winter in Japan.
At one sports arena in Minamisanriku where 1,500 evacuees are staying, old people crowded at a counter stacked with pills and bandages, while about 30 people slept on beds or on the floor in a makeshift clinic with doctors on standby.
"It's less a problem of medical supplies now, but a problem of finding out what medicine is lacking where and centralizing that information," said Nobuyuki Maki, a doctor.
"Many places in this area haven't restored mobile phone connections yet so there are still problems with communication."
(Additional reporting by Paul Eckert, Shinichi Saoshiro and Raju Gopalakrishnan in Tokyo, Jon Herskovitz and Chisa Fujioka in Minamisanriku; Frederik Dahl and Sylvia Westall in Vienna; Lisa Richwine in Washington; Alister Doyle in Oslo; Christopher Doering in Washington; Jonathan Standing in Taiwan; Writing by Andrew Cawthorne; Editing by Robert Birsel)