By Michael Shields and Fredrik Dahl
VIENNA (Reuters) - The radioactive contamination of some food near Japan's stricken nuclear plant has become a concern even as authorities report progress in their battle to prevent a meltdown at the site, U.N. officials said on Sunday.
"There have been some positive developments in the last 24 hours but overall the situation remains very serious," said Graham Andrew, a senior official of the International Atomic Energy Agency (IAEA).
He was speaking at a news conference after Japan restored power to some units at the crippled nuclear reactor in a race to avert disaster at the plant wrecked by an earthquake and tsunami that hit Japan on March 11, killing thousands of people.
In addition, workers have successfully placed two other reactor units into "cold shutdown," with cooling systems stable and under control, the IAEA said.
"Japanese authorities have notified the IAEA of progress at the Fukushima Daiichi nuclear power plant," the Vienna-based nuclear agency said in a statement on its website.
But cases of contaminated vegetables, dust and water have raised new fears in Japan. The government has prohibited the sale of raw milk from Fukushima prefecture and spinach from a nearby area. It is considering further restrictions on food.
The IAEA confirmed that in some areas iodine 131 -- a radioactive isotope -- had been found in milk and freshly grown vegetables "significantly above the levels set by Japan for restricting consumption of these food products," Andrew said.
"The contamination of food and water is a concern," another IAEA official, Gerhard Proehl, told the news conference. He said the situation must be monitored carefully.
Measurements at a location about 46 km from the plant showed iodine 131 levels in milk up to 15 times above the level suitable for infants, he said. For spinach, detected levels were also many times above the limit.
Proel said Iodine 131 decays quickly and is not a long-term concern, but another radioactive element called caesium 137 "could stay in the soil for quite a long time" and could potentially be absorbed by plants.
Andrew described the food contamination as "a very localized phenomenon at the moment as far as we know" and also said that food produced in other countries had not been affected.
Asked whether there may be any trade implications, U.N. food agency official David Byron said national authorities were responsible for food production, distribution and sale.
"That doesn't mean the importing countries aren't doing anything as well," said Byron, who is currently seconded to the IAEA. "One way or another it is either being controlled by the Japanese, which I assume aren't shipping anything at this point, or being monitored ... by the importing control authority."
Last week, Japanese authorities recommended that people who have evacuated the area near the Fukushima plant should take stable iodine as a precaution, the IAEA said on Saturday.
Taken as pills or syrup, stable (non-radioactive) iodine can be used to help protect against thyroid cancer in the event of radiation exposure in a nuclear accident.
(Editing by Kate Kelland)