Engineers have restored electricity to all six reactors at the crippled Fukushima nuclear power plant in Japan.
They have started a water pump at one of them to reverse the overheating, in the first clear signs of progress in tackling the world's worst atomic crisis in 25 years.
Japan suffered an estimated $250bn in damage from the earthquake and tsunami on 11 March.
More than 21,000 people are dead or missing, while the 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 ten-day nuclear crisis was over, said: 'Well, we believe so, but I don't want to make a blanket statement.'
Nuclear Regulatory Commission Chairman Gregory Jaczko added that radiation levels at the plant appeared to be falling.
However, the situation is far from under control.
Japan's nuclear safety agency said pressure is rising in the most threatening reactor - No 3 - which contains highly toxic plutonium.
Some workers were forced to leave the plant today after smoke was seen rising from that reactor.
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 today.
Cases of contaminated vegetables and milk have already stoked anxiety despite assurances from officials that the levels are not dangerous. The plant lies 240km north of Tokyo.
The government has now banned the sale of raw milk from Fukushima prefecture and might announce more extensive restrictions on food this week.
'The contamination of food and water is a concern,' said Gerhard Proehl of the Vienna-based UN watchdog the International Atomic Energy Agency (IAEA).
Meanwhile, 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,' Mr 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 were closed for a holiday today.
