Engineers restored electricity to three reactors at a crippled Japanese nuclear power plant and hope to test water pumps at the quake-damaged facility soon, the first clear signs of progress in tackling the world's worst atomic crisis in 25 years.

Japan suffered an estimated $250 billion in damage from the earthquake and tsunami on March 11 that left more than 21,000 people dead or missing, while radiation leaks from the Fukushima nuclear plant have caused global alarm.

US Energy Secretary Steven Chu, asked by CNN whether the worst of Japan's 10-day nuclear crisis was over, said: "Well, we believe so, but I don't want to make a blanket statement."

US Nuclear Regulatory Commission Chairman Gregory Jaczko added that radiation levels at the plant appeared to be falling.

However, the situation was far from under control on Monday.

Japan's nuclear safety agency said pressure was rising in the most threatening reactor, No. 3, which contains highly toxic plutonium, and this may have to be released by "venting" steam, a step taken last week that discharged low levels of radiation into the atmosphere.

Other problems have also emerged, especially from contaminated food and water.

The health ministry urged some residents near the plant in Fukushima prefecture to stop drinking tap water after high levels of radioactive iodine were detected, Kyodo news agency said on Monday.

Cases of contaminated vegetables and milk have already stoked anxiety despite assurances from officials that the levels are not dangerous. The plant lies 240 km (150 miles) north of Tokyo.

The government prohibited the sale of raw milk from Fukushima prefecture and spinach from another nearby area and might announce more restrictions on food on Monday.

"The contamination of food and water is a concern," said Gerhard Proehl of the Vienna-based U.N. watchdog the International Atomic Energy Agency (IAEA).

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

Japan's crisis spooked markets last week, prompted rare intervention by the G7 group of rich nations to stabilise the yen, and fuelled concerns the world economy may suffer because of disrupted supplies to the auto and technology industries.

Tokyo's markets are closed for a holiday on Monday. The Nikkei index shed 10 percent last week, wiping $350 billion off market capitalisation.

No power yet to reactors 1, 3 and 4

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.

"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 IAEA.

Working in suits sealed by duct tape, engineers have connected power cables to the No. 2, 5 and 6 reactors and plan to start testing systems soon, officials say. Officials earlier said a cable had been connected to reactor No.1 but engineers on Monday said this was not yet the case.

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

If the pumps cannot restart, drastic and lengthy measures     may be needed like burying the plant in sand and concrete.

Some expatriates, tourists and local residents have fled Tokyo over radiation fears. Those who remain are subdued but not panicked.

"There's no way I can check if those radioactive particles are in my tap water or the food I eat, so there isn't much I can really do about it," said Setsuko Kuroi, an 87-year-old woman shopping in a supermarket with a white gauze mask over her face.

Aid trickle

Easing Japan's gloom briefly, local TV on Sunday showed one moving survival tale: an 80-year-old woman and her 16-year-old grandson rescued from their damaged home after nine days.

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.

Scores of nations have pledged aid to victims, but little foreign relief supplies are visible in some devastated towns and villages.

"All we have had is the clothes on our backs. But they are good enough. They've kept us warm through all of this," said Machiko Kawahata as she, her daughter and granddaughter looked for clothes at a drop-off point in Kamaishi, a coastal town. "We will make do and we will make it through this."

The 9.0-magnitude quake and ensuing 10-metre (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.

While Japanese have been focused on the rescue operation rather than recriminations, media and others have raised questions over the government and plant operator Tokyo Electric Power Company's (Tepco) performance.

There have been suggestions the nuclear drama was taking priority over the human suffering, and that the early response of some officials was slow and opaque.

Tepco head Masataka Shimizu apologised at the weekend for "causing such trouble" at the plant but has not visited the site or made a public appearance in a week. – Reuters