People watch a television broadcasting Japan's Emperor Akihito's address to the nation in Tokyo. Japanese Emperor Akihito said on Wednesday problems at Japan's nuclear-power reactors were unpredictable and he was "deeply worried". (ISSEI KATO/REUTERS)

The nuclear power plant crisis in Japan will probably take weeks to resolve, forcing Japanese workers to intensify their risky efforts to bring the stricken Fukushima Daiichi nuclear power plant under control, a top U.S. official said Thursday.

“This is something that will likely take some time to work through, possibly weeks, as eventually you remove the majority of the heat from the reactors and then the spent-fuel pools,” Nuclear Regulatory Commission Chairman Gregory Jaczko told reporters at a White House briefing. “So it’s something that will be ongoing for some time.”

Jaczko’s statement came after Japanese military and police personnel risked exposure to dangerous levels of radiation to use helicopters and water canons to douse unit 3 at the plant with thousands of gallons. Reports of steam rising from one of the stricken reactors indicated that the dramatic efforts had delivered at least some water. But it remained unclear whether it was enough to keep radiation levels from spinning out of control.

“It hasn’t gotten worse, which is positive,” Graham Andrew of the International Atomic Energy Agency (IAEA) said. “The situation remains very serious but there has been no significant worsening since yesterday.”

Tokyo Electric Power Co., which owns the plant, said radiation levels had dropped following the hour-long effort. But Andrew, a senior aide to IAEA Director General Yukiya Amano, cautioned at a news conference: “It is still possible that it could get worse.”

At the same time, Tokyo Electric reached unit 2 at the facility with a new electric cable in the hopes of restoring power and restarting the plant’s on-site cooling system using seawater. Power had not yet been reactivated to the facility, but officials said they hoped to connect the line, perhaps as early as Friday, after they had finished spraying water at the nearby unit 3 reactor.

President Obama, addressing the crisis in a statement issued from the Rose Garden, said, “We are working aggressively to support our Japanese ally. “ He reiterated assurances that the disaster posed no threat to the United States, adding he had instructed the Nuclear Regulatory Commission to review the safety of U.S. nuclear plants in the wake of the crisis. There was no reason to believe they were unsafe, he stressed.

“Our nuclear plants have undergone exhaustive study and been declared safe,” Obama said.

Obama also made an unannounced visit to the Japanese Embassy and signed a condolence book.

“My heart goes out to the people of Japan during this enormous tragedy. Please know that America will always stand by one of its greatest allies during this time of need,” Obama wrote. “Because of the strength and wisdom of its people, we know that Japan will recover, and indeed will emerge stronger than ever. And as it recovers, the memory of those who have been lost will remain in our hearts, and will serve only to strengthen the relationship between our two countries. May God bless the people of Japan.”

Obama then told reporters he hoped to communicate how “heartbroken” America is over the tragedy.

A U.S. C-17 military plane had landed in Japan carrying a team of 33 and 17,000 pounds of supplies, including equipment to monitor air measurements for radiation, Deputy Energy Secretary Daniel Poneman told reporters earlier in the day.

U.S. diplomats in Tokyo have dispatched 14 buses to evacuate Americans who have been stuck north of Sendai, unable to evacuate because of the lack of transportation, said Patrick Kennedy, undersecretary for management at the State Department.

The Americans “have not been able to move south to Tokyo because of the absence of transportation, and they have not been able to move north towards Misawa on the northern tip because--again, absence of transportation and becuase of severe damage to the roads,” Kennedy told reporters on Thursday.

He said the buses, which can carry about 600 people, would travel back to Tokyo. The stranded Americans were outside the 50-mile zone surrounding the damaged nuclear power plant that the U.S. government has recommended that its citizens vacate.

The State Department on Thursday sent the first of what is expected to be several charter flights to evacuate the families of U.S. diplomatic personnel and other Americans wishing to leave, Kennedy said.

In a dangerous emergency mission, two Japanese military helicopters dropped more than 65 tons of water on the plant, apparently focusing on delivering water to a spent-fuel storage pool in unit 3. Then, soldiers used 11 high-pressure fire trucks designed for putting out fires at plane crashes to douse a damaged reactor building from a distance. The soldiers acted after Japanese police failed in their efforts to spray the building with water cannons normally used for riot control.

Members of the Self-Defense Force, as the military is known, moved their trucks into position and began to spray water Thursday evening, taking aim at the same unit 3 reactor that was targeted by helicopters earlier in the day. They sprayed water for more than half an hour before leaving the plant.

A spokesman for Tokyo Electric said radiation levels showed a very small decrease after the helicopter missions. But, noting the minuscule drop in radiation readings, the World Nuclear Association said the water drops by helicopter “did not appear accurate enough to be effective,” adding that “the effect at present seems marginal at best.” The London-based organization, which promotes nuclear energy, said one attempt was made to douse the unit 4 reactor building but that the pilots withdrew “after encountering high levels of radiation.”

The IAEA later said unit 4 remains a major safety concern. It said there was no information on the level of water in the pool at unit 4 used for cooling spent nuclear fuel.

As long as conditions are not deemed too dangerous, authorities are expected to continue with more helicopter drops and water-spraying ground missions Friday.

The growing crisis at Fukushima Daiichi prompted U.S. officials to contradict Japan’s recommendations and urge Americans to stay at least 50 miles from the plant — four times the distance recommended by the Japanese government. While Japan did not change its recommended evacuation area, a 12.4-mile radius around the plant, several other nations joined the United States in adopting a 50-mile recommended radius, including Canada, Britain and South Korea.

The Pentagon said Thursday that it has sent a nine-member team of radiological specialists to Japan from the Colorado-based U.S. Northern Command to advise the Japanese military on responding to nuclear hazards.

As Japan struggled to avert a nuclear disaster, ripple effects from the crisis spread to other countries in the region.

In China, panic-buying swept from the country’s eastern coast all the way to Beijing, with residents rushing to stores to stock up on salt. People apparently believe the iodine in salt will protect them from radiation; others feared that sea salt would become scarce if the East China Sea becomes contaminated because of Japan’s nuclear plant crisis.

In Beijing and elsewhere, several supermarkets also ran out of imported milk powder, soybean sauce and instant noodles, as people stocked up on provisions even as the government issued repeated assurances that there was no radiation threat to China.

“I don’t know when I can replenish our stock,” said Chen Zhonghai, manager of Jinli Super Market in Wenzhou City. “The residents worry that the salt produced in the future will be contaminated and can’t be eaten.  It’s totally unnecessary.”

The nuclear plant woes were also affecting people across Japan, as energy demands taxed the limited supply. The Japanese government warned that because of cold weather and an increase in power usage, Tokyo was subject to an extensive blackout Thursday night that could leave much of the world’s most-populated metropolitan area in the dark.

“If tonight we see another power consumption situation just like this morning, it could lead to a wide-scale and unpredictable power outage,” said Banri Kaieda, the country’s trade minister.

Later, however, officials said the threat of a blackout in Tokyo might have passed, because people heeded warnings and were conserving enough energy.

The energy shortage stems from the mounting problems with the nation’s power plants, several damaged in the 9.0-magnitude earthquake and tsunami that devastated the country last Friday.

Meanwhile, Japan’s death toll continued to rise. According to the National Police Agency, 5,429 people have died and an additional 9,594 are still missing. The list of casualties is expected to eventually top 10,000.

Also Thursday, the yen leaped to a record-high 76.25 against the U.S. dollar after trading in the low 80s in the days before the earthquake and tsunami. The Tokyo Stock Exchange was down, however, as the Nikkei fell 131 points and ended the day at 8962.

In Washington, Jaczko painted a bleak picture of the crisis, saying in congressional testimony Wednesday that a deep pool in unit 4 holding uranium fuel at the Fukushima Daiichi facility sat empty of water needed to prevent releases of radiation. “And we believe that radiation levels are extremely high,” he added.

That assessment was rejected Thursday by the Tokyo Electric spokesman, who said an aerial survey showed that the fuel pool at the unit 4 reactor still contained water. The spokesman could not say how much water was in the pool, but he said no rods were exposed.

However, an official with Japan’s nuclear safety agency later expressed skepticism about that claim and moved closer to the U.S. position, AP reported. “Considering the amount of radiation released in the area, the fuel rods are more likely to be exposed than to be covered,” the official, Yuichi Sato, said.

The question is an important one: If exposed to the air, the spent-fuel rods would start to decay and release radioactivity into the air.

Getting a close look at the damage sustained by the plant’s six units has proven to be difficult and dangerous. The United States has offered the use of its Global Hawk unmanned drone planes, a military aircraft often used for surveillance.

Obama spoke with Japanese Prime Minister Naoto Kan by phone Thursday morning (Wednesday night in Washington), and the White House said Obama emphasized that the United States would do “everything possible to support Japan in overcoming the effects of the devastating earthquake and tsunami.” Kan briefed Obama on steps taken to contain the nuclear crisis, the White House said.

With Japan’s northeastern coastline ravaged and fears of radiation growing, Emperor Akihito made rare public remarks Wednesday, saying he was “deeply concerned about the nuclear situation.”

The emperor’s televised address — his first at a time of national crisis — underscored the gravity of the moment and highlighted the myriad problems still plaguing Japan nearly a week after the 9.0-magnitude earthquake and tsunami struck: a death toll that grows by the day; conflicting safety and evacuation information; growing distrust by locals and foreigners who call Japan home; a scarcity of gas, food and other resources; and the difficulty some aid workers have had delivering supplies.

Maese reported from Tokyo. Correspondents Chico Harlan in Tokyo and Keith B. Richburg in Beijing and staff writers Mary Beth Sheridan, Brian Vastag and David A. Fahrenthold in Washington contributed to this report.