OSAKA, Japan — Japan's nuclear crisis deepened on Thursday after fresh attempts to cool a stricken reactor met with mixed results, raising anxiety levels among millions of people in the region, and as far away as Tokyo.

The day began with dramatic live footage of Japanese military helicopters, manned by crews in protective suits, dropping seawater by the tons over the plant's No. 3 reactor.

The efforts marked a new, and more desperate, tactic in Japan's fight to save the plant from a potentially catastrophic meltdown.

All six reactors at Fukushima Daiichi atomic power plant have developed serious cooling problems since vast stretches of Japan's northeast coast were devastated by a powerful earthquake and tsunami last Friday.

In the evening Thursday, five trucks equipped with high-pressure water cannons normally used to quell rioters sprayed tons of water at the reactor building in an attempt to cool a storage pool for spent fuel rods that has lost water and appears to be overheating.

If the worst happens, overheating fuel rods could begin to melt, releasing high levels of radiation.

The threat of a serious nuclear meltdown is fraying nerves in Tokyo, more than 150 miles south of the disabled power plant.

There is no exodus from the capital — yet — but fuel is being rationed and supermarket shelves are quickly emptied of rice, milk, noodles and other foods. Stores report a surge in demand for survival staples such as torches, batteries and sleeping bags.

Train services have been disrupted by voluntary power cuts and many employees have been told to sit out the crisis at home. A rush of transactions led to the sudden closure of thousands of ATMs at Mizuho Bank.

For all their stoicism, Tokyoites are preparing for the worse. Long lines formed outside the city's passport office, while bullet trains are filling up with a steady stream of expatriates, women and children. Television footage showed hundreds of people leaving the city on buses.

Authorities warned that Tokyo could experience a blackout on Thursday evening, but it was granted a reprieve after the operator of the plant, Tokyo Electric Power Co. (TEPCO), said demand had been lower than expected, thanks to energy-saving efforts by businesses and households.

The company claimed a victory of sorts, saying the 30 tons of water aimed at the No. 3 reactor had succeeded in cooling the storage pool. But it added that it had observed little change in radioactivity levels at the facility.

The helicopters and water cannons will resume water-bombing the plant on Friday, the firm said.

The team of technicians, firefighters and soldiers face myriad problems, but the aim is clear: to cool the No. 3 reactor and replenish the pool containing spent fuel rods.

TEPCO is unable to get close enough to take accurate readings, but it is working on the chilling assumption that the pool is almost empty, raising the risk that the fuel rods will overheat and melt.

The firm is also attempting to open a temporary power line to the plant that would allow it to pump water directly into the storage pools and reactor cores.

Japanese authorities' inability to contain the damage almost a week after the tsunami coincided with a grim prognosis from nuclear experts in the United States.

Gregory Jackzo, chairman of the U.S. nuclear regulatory commission, told a congressional hearing in Washington that the storage pool at another reactor had lost all of its water and was in danger of releasing more radioactive material.

As a result, a growing number of foreign governments are taking steps to remove their citizens from danger.

The U.S. imposed an evacuation zone within a 50-mile radius of the plant and said it would send chartered aircraft to take Americans out of the country.

"We are recommending, as a precaution, that American citizens who live within 80 kilometers of the Fukushima nuclear power plant evacuate the area or to take shelter indoors if safe evacuation is not practical,'' the U.S. Embassy said in a statement.

On Thursday, Obama made an unannounced visit to the Japanese Embassy, signing a condolence book for victims of recent disasters in Japan.

According to reports, he commented on "how heartbroken the American people are" and defended his authorization of the advice for U.S. citizens to evacuate the immediate area, owing to an "an obligation to educate Americans."

Obama also said during his visit to the embassy that the U.S. felt "great urgency" to assist Japan and was confident that Japan will rebuild, according to news reports.

Britain and other countries have imposed the same exclusion zone, which is far wider than the 30-kilometer radius established by the Japanese government.

Britain's foreign office has also told U.K. citizens to "consider leaving" Tokyo and northeast Japan in light of the "evolving situation" in Fukushima and the disruption to life in Tokyo.

France and Australia have been less equivocal, advising citizens to leave the area. Russia said it was pulling out the families of diplomats, while Hong Kong urged its citizens to leave Japan or head to the country's southern regions.

Several businesses have taken similar measures, and some airlines are rerouting flights away from Tokyo to other Japanese airports until at least the weekend.

Asked about the wider evacuation zone favored by the United States, the government's top spokesman, Yukio Edano, said: "It is understandable for foreign governments to act conservatively to protect their citizens, so we should react to the news calmly."

The White House said there was no suggestion of a rift developing between Washington and Tokyo over the latter's handling of the crisis. "I will not from here judge the Japanese evaluation of the data," spokesman Jay Carney told reporters. "This is what we would do if this incident were happening in the United States."

The nuclear crisis has managed to overshadow the huge humanitarian effort in northeastern Japan, where evacuees and aid workers contended with snowfall and plummeting nighttime temperatures.

More than 5,300 people are officially listed as dead, but officials believe the total will rise to well over 10,000.

Nearly a week after the disaster, more than 452,000 people are staying at schools and other shelters, the already harsh conditions compounded by shortages of fuel, medicine and water.