Japan's earthquake-stricken nuclear complex is still emitting radiation but the source is unclear, a senior U.N. atomic agency official said, as workers made progress restoring electric power to the site.
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The International Atomic Energy Agency (IAEA) also raised concerns about a lack of information from Japanese authorities, as workers battling to cool the nuclear reactors faced rising temperatures around the core of one reactor.
"We continue to see radiation coming from the site ... and the question is where exactly is that coming from?" James Lyons, a senior official of the International Atomic Energy Agency (IAEA), told a news conference in Vienna on Tuesday.
Despite hopes of progress in the world's worst nuclear crisis in a quarter of a century, triggered by an earthquake and tsunami that left at least 21,000 people dead or missing, plant operator Tokyo Electric Power Co (TEPCO) said it needed more time before it could say the reactors were stabilized.
Senior IAEA official Graham Andrew said that the overall situation remained "very serious" and that the U.N. atomic watchdog was concerned it had not received some information from Japan about the Fukushima nuclear plant.
"We have not received validated information for some time related to the containment integrity of unit 1. So we are concerned that we do not know its exact status," he said.
The IAEA also lacks data on the temperatures of the spent fuel pools of reactors 1, 3 and 4, he said, though Japan was supplying other updates.
Technicians working inside an evacuation zone around the plant on Japan's northeast Pacific coast, 250 km (150 miles) north of Tokyo, have attached power cables to all six reactors and started a pump at one to cool overheating nuclear fuel rods.
Local media reported late on Tuesday that lighting had been restored at one of the control rooms, bringing the operators a step closer to reviving the plant's cooling systems.
Earlier smoke and steam were seen rising from two of the most threatening reactors, No.2 and No.3, threatening to dash hopes of progress in bringing them under control.
There have been several blasts of steam from the reactors during the crisis, which experts say probably released a small amount of radioactive particles.
Hidehiko Nishiyama, the deputy-director general of Japan's nuclear safety agency, later said the smoke at reactor No.3 had stopped and there was only a small amount at No.2.
He gave no more details, but a TEPCO executive vice president, Sakae Muto, said the core of reactor No.1 was now a worry with its temperature at 380-390 Celsius (715-735 Fahrenheit).
"We need to strive to bring that down a bit," Muto told a news conference, adding that the reactor was built to run at a temperature of 302 C (575 F).
Asked if the situation at the problem reactors was getting worse, he said: "We need more time. It's too early to say that they are sufficiently stable." Reuters earlier reported that the Fukushima plant was storing more uranium than it was originally designed to hold, and that it had repeatedly missed mandatory safety checks over the past decade, according to company documents and outside experts.
Questions have also been raised about whether TEPCO officials waited too long to pump sea water into the reactors and abandon hope of saving the equipment in the aftermath of the earthquake and tsunami.
But one expert said the smoke or steam seen over the reactors did not seem to be linked to rises in radiation levels.
"Overall there is progress compared to a few days ago when everything seemed hopeless. But we still judge the situation to be critical," said Per Bystedt, an analyst at the Swedish Radiation Protection Authority.
"The positive thing is that electric power is more or less connected to all the plants."
Away from the plant, mounting evidence of radiation in vegetables, water and milk stirred concerns in Japan and abroad despite officials' assurances that the levels were not dangerous.
TEPCO said radiation was found in the Pacific Ocean nearby, not surprising given rain and the hosing of reactors with sea-water. TEPCO officials have said some of the water from the hosing was spilling into the sea.
Radioactive iodine in the sea samples was 126.7 times the allowed limit, while caesium was 24.8 times over, the Kyodo news agency said. That still posed no immediate danger, TEPCO said.
"It would have to be drunk for a whole year in order to accumulate to 1 millisievert," a TEPCO official said, referring to the standard radiation measurement unit.
People are generally exposed to 1-10 millisieverts a year from background radiation caused by substances in the air and soil.
The Health Ministry said residents of five municipalities in Fukushima should not use tap water for baby powder milk after the water was found to have more than the standard level of radioactive iodine allowed for babies. Authorities have also stopped shipments of milk and some vegetables from the area.
Despite the warnings, experts say readings are much lower than around Chernobyl after the 1986 accident in Ukraine.
Japan is a net importer of food, but also exports fruit, vegetables, dairy products and seafood, with its biggest markets in Hong Kong, China and the United States.
Japan's neighbors including China, South Korea, Taiwan and Thailand, are monitoring Japanese food imports. Australia's food regulator said the risk was negligible and no extra restrictions on Japanese food were in place.
U.S. Defense Secretary Robert Gates said on Tuesday he was concerned about radioactive fallout affecting the 55,000 troops in and around Japan, many involved in a massive relief operation for Washington's close ally. "We're watching it very carefully. We're very concerned about the health of our men and women in uniform," he said.
"But we're also deeply concerned about the well being of our Japanese allies. So we will do what's best both for our men and women in uniform and our alliance," Gates told reporters in Moscow.
Miniscule numbers of radioactive particles believed to have come from the crippled nuclear power plant have been detected as far away as Iceland, diplomatic sources said on Tuesday.
They stressed the tiny traces, measured by a network of international 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," Andreas Stohl, a senior scientist at the Norwegian Institute for Air Research, said. "Over Europe there would be no concern about human health."
Nevertheless, Italy's Industry Minister Paolo Romani said on Tuesday the government will announce a one-year moratorium on site selection and building of nuclear power plants.
A poll of 814 Americans conducted last week by ORC International for the Civil Society Institute think tank and released on Tuesday found 53 percent would support a moratorium on new nuclear reactor construction if the country was able to meet its energy demand through increased efficiency and renewable sources such as wind and solar.
COSTLIEST NATURAL DISASTER
The prospects of a nuclear meltdown in the world's third-biggest economy -- and its key position in global supply chains, especially for the automobile and technology sectors -- rattled investors worldwide last week and prompted rare joint currency intervention by the G7 group of rich nations.
Investors in Tokyo stocks took heart from signs of progress at the plant, with the main index jumping more than 4 percent after a holiday on Monday.
The yen edged up, putting traders on heightened alert for more central bank intervention.
Damage from the earthquake and tsunami is estimated at $250 billion, making it the world's costliest natural disaster.
The official death toll exceeded 9,000, but with 12,654 people reported missing, it is certain to rise. Police say more than 15,000 people probably died in Miyagi prefecture, one of four that took the brunt of the tsunami.
The quake and tsunami obliterated towns and left more than 350,000 people homeless.
Fuel shortages, icy rain and power outages have hampered efforts to help survivors but relief workers reported some progress as mangled roads reopened and new homes were built.
Still, 2.4 million people are without access to water and 221,000 households are without power, while many people are still searching for loved ones. "They found my brother-in-law's body yesterday but they can't find my younger sister," said Tomiko Oikawa, 77, who was camped out at a sports arena-turned-evacuation center in Minamisanriku town.
"Her house was washed away. She may be gone but I want to at least find her body. I think, should I give up, but I keep looking for her."