Abnormal levels of radiation in milk and spinach have been detected near a stricken nuclear plant, but the foods pose no immediate threat to humans, government spokesman Yukio Edano said Saturday.

Traces of radioactive iodine were meanwhile found in tap water in Tokyo and several prefectures near the atomic power complex, a science ministry official said, but the levels were well below the legal limit.

The findings are nevertheless likely to fuel consumer fears in the wake of last week's quake and tsunami, which critically damaged the Fukushima No.1 plant northeast of Tokyo, sending radioactive substances leaking into the air.

"Radiation exceeding the limit under Japanese law was detected," Edano told reporters.

The contaminated milk was found in Fukushima prefecture, where the quake-damaged atomic power station is located, while the tainted spinach was discovered in neighbouring Ibaraki prefecture, Edano told reporters.

The milk was found more than 30 kilometres (20 miles) from the Fukushima No. 1 nuclear power plant -- outside the government's exclusion zone.

The spokesman said the health ministry had ordered authorities in both prefectures to check where the products came from, how they were distributed and -- depending on their findings -- suspend sales.

"The government will do its utmost... to avoid health hazards and to resolve this problem," Edano said. "The number does not present an immediate health threat. I would like to ask you to act calmly."

He noted that even if a consumer were to drink the contaminated milk for a year, the radiation level would be the equivalent of one hospital CT scan.

Plant operator Tokyo Electric Power Co (TEPCO) apologised for the contamination of foodstuffs and said it would look into compensating the farmers affected, Jiji Press reported.

Hideki Mukaitsubo, president of Fukushima Prefecture Minami Dairy located in Izumizaki village, 60 kilometres from the nuclear plant, said he was concerned about the long-term prospects for the industry.

"We've halted shipping completely. I really don't know what to do from tomorrow," Mukaitsubo told AFP by telephone, criticising the government's "unclear" sampling methods.

"I don't know any more about my future," said Mukaitsubo, who takes milk from several local producers.

Yukihiro Ebisawa, an official at Japan Agricultural Cooperatives in Ibaraki prefecture, said they had received an order to tell spinach producers not to harvest or ship any more produce.

"We are worried... I hope the situation will calm down as soon as possible," Ebisawa told AFP.

On Thursday, Japan instructed local authorities to start screening food for radioactivity following a series of accidents at the plant 250 kilometres (155 miles) northeast of Tokyo.

It is the first time Japan has set legal radiation limits on domestically produced foodstuffs.

The guidelines vary depending on the product and type of radioactive substances, and were set in consideration of internationally accepted levels and average intake in the Japanese diet.

Abnormal levels of radioactive iodine were found in the water supply in Tokyo as well as the central prefectures of Gunma, Tochigi, Saitama, Chiba and Niigata, according to the science ministry official.

The March 11 quake and tsunami knocked out the reactor cooling systems at the nuclear complex, which led to a string of explosions and fires.

Radioactive substances have since leaked into the air and workers are now battling to restore power to the plant and get the cooling systems running.

Several Asian nations have said they will screen food imported from Japan for radiation, while the European Union has called for similar checks.

Japanese web users were quick to react to the announcement on popular micro-blogging service Twitter -- and their words for the government and Prime Minister Naoto Kan were anything but kind.

"What the prime minister can do is to go out in the field and eat this spinach and milk, this news will likely spark harmful rumours," said one with the username sakuya_ntg.

"This spinach and milk problem, I can't make head nor tail of it because there is no information on the sampling... or whether these two are the only positive results after lots of tests," said another, tweetingMiki.
"I am waiting for details."