The world's worst atomic crisis in 25 years playing out 240 km (150 miles) north of Tokyo was triggered by a huge earthquake and tsunami that left at least 21,000 people dead or missing.

Technicians working inside an evacuation zone around the stricken plant on Japan's northeast Pacific coast have attached power cables to all six reactors and started a pump at one of them to cool overheating nuclear fuel rods.

"We see a light for getting out of the crisis," an official quoted Prime Minister Naoto Kan as saying, allowing himself some rare optimism in Japan's toughest moment since World War Two.

Kyodo news agency said steam appeared to be rising from reactor No. 2 and white haze was detected above reactor No. 3. There have been several blasts of steam from the reactors during the crisis, which experts say probably released a small amount of radioactive particles.

Plant operator Tokyo Electric Power Co (TEPCO) said later the smoke had turned to steam and it was deemed safe to continue work in bringing the plant under control.

Away from the plant, mounting evidence of radiation in vegetables, water and milk stirred concerns among Japanese and abroad despite assurances from Japanese officials that the levels were not dangerous.

TEPCO said radiation was found in the Pacific ocean nearby , not surprising given rain and the hosing of reactors with seawater. Some experts said it was unclear where the used seawater was ultimately being disposed.

Radioactive iodine in the sea samples was 126.7 times the allowed limit, while cesium was 24.8 times over, Kyodo said. That still posed no immediate danger, TEPCO said.

"It would have to be drunk for a whole year in order to accumulate to one millisievert," a TEPCO official said, referring to the standard radiation measurement unit. People are generally exposed to about 1 to 10 millisieverts each year from background radiation caused by substances in the air and soil.

Japan has urged some residents near the plant to stop drinking tap water after high levels of radioactive iodine were detected. It has also stopped shipments of milk, spinach and another vegetable called kakina from the area.

Experts say readings are much lower than around Chernobyl after the 1986 accident in Ukraine.

The World Health Organization (WHO) said the radiation impact was more serious than first thought, when it was expected to be limited to 20-30 km (12-19 miles) from the plant.

However, Peter Cordingley, spokesman for the WHO's regional office, told Reuters there was no evidence of contaminated food reaching other countries from the Fukushima complex.

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

China said it was monitoring food imports from Japan. South Korea is expanding inspection of Japanese food and in Taiwan, one Japanese restaurant is offering diners a radiation gauge in case they are nervous about the food.

PLANT SITUATION MAY BE STABILISING

Experts said the situation appeared to be improving at the Fukushima plant, especially after more advances than setbacks in the past 48 hours.

"Things are beginning to trend in the right direction. TEPCO will need to get electrical power back on line to all six reactors and they will have to make sure that components are working," said Mark Prelas, director of research for the Nuclear Science and Engineering Institute at the University of Missouri.

The priority on Tuesday will be patching up the No. 2 reactor so electricity, already connected, can be turned on. By Tuesday morning, reactor No. 1 was receiving power. Reactor No. 5 has both electricity and working cooling pumps.

The U.S. Nuclear Regulatory Commission said the crisis appeared close to stabilizing.

The prospects of a nuclear meltdown in the world's third-biggest economy -- and its key position in global supply chains, especially for the automobile and technology sectors -- rattled investors worldwide last week and prompted rare joint currency intervention by the G7 group of rich nations.

Tokyo stocks were up at 3 percent at 0220 GMT after a holiday on Monday, following a rise in global stocks as progress in the nuclear crisis revived risk appetite. The yen slid on speculation of more G7 intervention.

Damage from the earthquake and tsunami is estimated at $250 billion, making it the world's costliest natural disaster.

DEVASTATED NORTH

The official death toll - 8,805 by Tuesday morning - is certain to rise, with 12,654 reported missing.

Police say more than 15,000 people probably died 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 obliterated towns, which are now wastelands of mud and debris, leaving more than 350,000 people homeless.

Japanese are famed for resilience though, and there was none of the chaos or looting that major global disasters often spark.

In one devastated northern town, Rikuzentakata, rebuilding has begun. Steel structures, with walls and wood floors, have been erected at a hilltop school, to provide temporary housing.

There is widespread admiration for the workers facing high radiation dosages on the front line at Fukushima. Some have wept with tension and relief after finishing their shifts.