As emergency crews struggled to stop further radiation leaks from Japan's crippled nuclear plant, senior UN officials said some progress was being made but warned that it was too early to pronounce that the worst was over.
High levels of radioactivity appeared to be contained to the immediate area of the stricken complex on Japan's northeastern coast.
Reporters briefed by the UN's International Atomic Energy Agency were shown charts showing readings well below health-threatening thresholds from cities a few hundred kilometres away from the Fukushima Dai-ichi plant. A diplomat with access to UN radiation tracking meanwhile described levels from Friday in Russia and California as minuscule.
Japanese officials said the crisis at the plant appeared to be stabilizing, with near-constant dousing of dangerously overheated reactors and uranium fuel, but the situation was still far from resolved - comments echoed by Graham Andrew, a senior aide to IAEA chief Yukiya Amano.
"The risk is reducing day by day," Andrew said. "Things are going in the right direction.
"But could we have something unexpected? Most certainly."
Amano was even more cautious, in comments to reporters at Vienna airport after a one-day trip to Tokyo to assess the situation.
"I have seen strengthened activities and felt a very strong commitment to fight against this incident and to recover safety ... and the fact is that activities are increasing," he said. But, "I don't think it is time to say that things are going in a good direction or not."
In Japan, officials said that contamination from the site had seeped into the food chain, with radiation levels in spinach and milk from farms near the tsunami-crippled facility exceeding government safety limits. But they insisted unrealistically huge quantities would have to be consumed before health risks loomed.
Minuscule amounts of radioactive iodine also were found in tap water Friday in Tokyo and elsewhere in Japan - although experts said none of those tests showed any health risks. The Health Ministry also said that radioactive iodine slightly above government safety limits was found in drinking water at one point Thursday in a sampling from Fukushima prefecture, the site of the nuclear plant, but later tests showed the level had fallen again.
Commenting on atmospheric readings hundreds of kilometres outside of the crisis zone in Japan and further afield, the Vienna-based diplomat said the measurements are 100 million to 1 billion times below health-threatening levels.
He said the measurements were taken at Takasaki, Japan, about 330 kilometres southwest of the accident site near the city of Fukushima; at Petropavlosk-Kamchatsky on Russia's Kamtchatka Peninsula; and at Sacramento, California.
The diplomat - who asked for anonymity because the CTBTO does not make its data public - was citing readings taken from the Vienna-based Comprehensive Test Ban Treaty Organisation. He said the instruments at the monitoring stations were sensitive enough to measure single atoms.
Set up to monitor all nuclear testing, the CTBTO's worldwide network of stations can also detect earthquakes, tsunamis and fallout from accidents such as the disaster on Japan's northeastern coast that was set off by a massive earthquake and a devastating tsunami eight days ago.
In the United States, federal and state officials said that the doses of radiation that a person normally receives from rocks, bricks, and the sun are 100,000 times the dose rates detected from Japan at a monitoring station in California and another in Washington state.
Graham, the IAEA official, suggested that - if the situation stabilizes or improves - global fallout would be minimal in comparison to the kind of radiation spewed out by the 1986 explosion and fire at the Ukrainian reactor at Chernobyl.
"I can't see ... major implications in comparison to what happened in Chernobyl," he said. "But if things were to get worse, that situation would change."
Even in Japanese cities just a few hundred kilometers (miles) from the nuclear complex, "the levels of radiation we measure there are well below what would be dangerous to human health," he said.
Closer to the site, tainted milk was found 30km away, while the spinach was collected between 80km and 100km to the south, Chief Cabinet Secretary Yukio Edano told reporters in Tokyo.
While the radiation levels exceeded the limits allowed by the government, Edano said that the products "pose no immediate health risk" and that more testing was being done on other foods.
