TOKYO - The World Health Organization said on Monday that radiation in food after an earthquake damaged a Japanese nuclear plant was more serious than previously thought, eclipsing signs of progress in a battle to avert a catastrophic meltdown in its reactors.

Engineers managed to rig power cables to all six reactors at the Fukushima complex, 240 km (150 miles) north of Tokyo, and started a water pump at one of them to reverse the overheating that has triggered the world's worst nuclear crisis in 25 years.

Some workers were later evacuated from one of the most badly damaged reactors when smoke briefly rose from the site. There was no immediate explanation for the smoke, but authorities had said earlier that pressure was building up at the No 3 reactor.

Smoke was also seen at the No 2 reactor.

The March 11 earthquake and tsunami left more than 21,000 people dead or missing and will cost an already beleaguered economy some $250 billion, making it the world's costliest ever natural disaster.

The head of the U.N. atomic agency said the nuclear situation remained very serious but it would be resolved.

"I have no doubt that this crisis will be effectively overcome," Yukiya Amano, director general of the International Atomic Energy Agency (IAEA), told an emergency board meeting.

"We see a light for getting out of the crisis," a Japanese government official quoted Prime Minister Naoto Kan as saying.

But news of progress at the nuclear plant was overshadowed by mounting concern that radioactive particles already released into the atmosphere have contaminated food and water supplies.

"Quite clearly it's a serious situation," Peter Cordingley, Manila-based spokesman for the World Health Organization's (WHO) regional office for the Western Pacific, told Reuters in a telephone interview.

"It's a lot more serious than anybody thought in the early days when we thought that this kind of problem can be limited to 20 to 30 kilometers ... It's safe to suppose that some contaminated produce got out of the contamination zone."

However, he said there was no evidence of contaminated food from Fukushima reaching other countries.

Fukushima is the world's worst nuclear accident since Chernobyl, but signs are that it is far less severe than the Ukrainian disaster.

"The few measurements of radiation reported in food so far are much lower than around Chernobyl in 1986, but the full picture is still emerging," Malcolm Crick, secretary of the U.N. Scientific Committee on the Effects of Atomic Radiation, told Reuters.

TAP WATER

Japan's health ministry has urged some residents near the plant to stop drinking tap water after high levels of radioactive iodine were detected.

Cases of contaminated vegetables and milk have already stoked anxiety despite assurances from officials that the levels are not dangerous. The government has prohibited the sale of spinach from all four prefectures near the plant and also banned selling of raw milk from Fukushima prefecture.

There were no major reports of contaminated food in Tokyo, a city of about 13 million people. City officials however said higher-than-standard levels of iodine were found in an edible form of chrysanthemum.

"From reports I have heard so far, it seems that the levels of radioactive iodine and caesium in milk and some foodstuffs are significantly higher than government limits," said Jim Smith, a specialist in earth and environmental sciences at Britain's Portsmouth University.

"This doesn't mean that consumption of these products is necessarily an immediate threat, as limits are set so that foodstuffs can be safely consumed over a fairly long period of time. Nevertheless, for foodstuffs which are found to be above limits, bans on sale and consumption will have to be put in place in the affected areas."

Japan is a net importer of food, but has substantial exports -- mainly fruit, vegetables, dairy products and seafood -- with its biggest markets in China and the United States.

China will monitor food imported from Japan, the Xinhua news agency said, citing the country's quality control watchdog. South Korea will expand radioactivity inspection to processed and dried agricultural Japanese food, from just fresh produce.

MELTDOWN

The prospects of a nuclear power plant meltdown in the world's third-biggest economy and its key position in global supply chains rattled investors worldwide last week and prompted rare joint currency intervention by the G7 group of rich nations to stabilize markets.

Moody's Investors Service said in a report that the downside risks from the crisis had increased over the past week for the economy, sovereign credit, banking, insurance and non-financial corporate sectors.

Tokyo's markets were closed for a holiday on Monday. The Nikkei index shed 10 percent last week, wiping $350 billion off market capitalization, and at one point had lost as much as 20 percent in value.

In a much-needed boost for the battered market, billionaire investor Warren Buffett said the earthquake and tsunami were an "enormous blow" but should not prompt the selling of Japanese shares. Instead, he called events a "buying opportunity".

"It will take some time to rebuild. But it will not change the economic future of Japan. If I owned Japanese stocks, I would certainly not be selling them," Buffett said during a visit to a South Korean factory run by a company that is owned by one of his funds.

SITUATION CRITICAL AT PLANT

At Fukushima, 300 engineers have worked around the clock inside an evacuation zone to contain the worst nuclear accident since Chernobyl, Ukraine, in 1986.

They have been spraying the coastal complex with thousands of tonnes of sea water so fuel rods will not overheat and emit more radiation. Hopes for a more permanent solution depend on electricity cables reactivating on-site water pumps at each of the six reactors.

The most badly damaged reactors are No 3 and 4, which were both hit by explosions last week.

Official tolls of dead and missing are rising steadily -- to 8,450 and 12,931 respectively.

The death toll could jump dramatically since police said they believed more than 15,000 people had been killed in Miyagi prefecture, one of four that took the brunt of the tsunami.

The 9.0-magnitude quake and ensuing 10-meter (32-ft) tsunami made more than 350,000 people homeless. Food, water, medicine and fuel are short in some parts, and near-freezing temperatures during Japan's winter are not helping.