The nuclear situation in Japan, though improving, remains very serious because it may take days to restore the cooling process to the nuclear reactors in critical condition at Fukushima power plant, officials said Sunday.

Engineers restored electricity to reactors number two, five and six, with similar service to reactor one not far behind. Five and six have nearly normalized cooling operations. But efforts to get coolant pumping into the other overheating reactors to avert a nuclear meltdown were stalled.

However, U.S. Energy Secretary Steven Chu said the worst of the crisis was over.

On Sunday, Japan advised villagers near Fukushima not to drink tap water because of high levels of radioactive iodine in the water. The warnings came a day after Japan banned the sale of milk and vegetables in the prefecture of Fukushima for the same reason.

The officials said the levels, though higher than allowable under regulations, were not dangerous even to harm humans. A spokesperson for the International Atomic Energy Association confirmed the high iodine levels, but said they had not been detected in food outside of Japan.

A nuclear safety official in Japan said that the country should have started giving potassium iodide to nearby residents right after there was an explosion in one of the reactors.

"We should have made this decision and announced it sooner," he said. "It is true that we had not foreseen a disaster of these proportions. We had not practiced or trained for something this bad. We must admit that we were not fully prepared."

The earthquake and tsunami that battered the reactors has killed nearly 10,000 people with the final death toll expected to exceed 15,000 people because 21,000 people remain missing. More than 350,000 people are unable to live in their homes due to various evacautions, including one in the 12-mile radius around the power plant.

An 80-year-old woman and her 16-year old grandson were recently found alive in the wreckage--nine days after the earthquake hit.

Reach deputy editor Paresh Dave here. Follow him on Twitter: @peard33.