TOKYO – Japanese officials took a series of early steps today to bring the Fukushima Dai-ichi nuclear power plant under control, but a week into the crisis it was becoming apparent that they were confronting a problem that would not be resolved quickly.

A top U.S. nuclear official warned that the emergency could continue for weeks, and President Obama tried to reassure the American public about the safety of nuclear power plants in the United States.

The moves reflected widening worries in Japan and the U.S. about the failure so far to contain radiation leaks from nuclear plants damaged in last Friday’s 9.0-magnitude earthquake and the devastating tsunami that followed.

Tokyo Electric Power Co., which owns the facility, said a risky mission using helicopters and water cannons Thursday to dump tons of water on the most troubled reactor had succeeded in reducing radiation levels. But Graham Andrew of the International Atomic Energy Agency cautioned at a news conference: “It is still possible that it could get worse.”

Japanese officials said they were planning to send additional helicopters and fire trucks back to the plant today, and would continue trying to deliver water to storage pools. Without water, spent fuel rods stored in the pools would start to decay and release radioactive matter into the air.

There also were hopes that Japan’s success in reconnecting electric power to another reactor would allow engineers to restart pumps that play an essential role in delivering coolant.

Over the next few weeks, radiation will continue to spew from the stricken plant at levels high enough to make it difficult for people to work there. What’s more, the facility itself has been ravaged by earthquakes, flooding and explosions that have torn much of the infrastructure — power lines, pumps and pipes — to shreds and scattered debris, making access even for robots challenging.

As Japan continued its grim recovery effort, the official death toll from the earthquake and tsunami reached 5,692 people this morning, with more than 9,500 others listed as officially missing. Nearly half a million are being housed in temporary shelters, and widespread power outages have left broad swaths of the country without adequate heat.

In Washington, Obama made an unannounced visit Thursday to the Japanese Embassy and signed a condolence book. Later, speaking from the Rose Garden, he said the United States was “working aggressively to support our Japanese ally.”

But this morning, the Yomiuri Shimbun, a leading Japanese newspaper, reported that Japan’s government had turned down an early U.S. offer of help with cooling fuel rods at the damaged nuclear reactors. The paper reported that the government and Tokyo Electric Power believed that they would be capable of restoring the cooling system.

That report appeared to conflict with a dispatch from The Associated Press that quoted Japanese government spokesman Yukio Edano as saying that “we are coordinating with the U.S. government as to what the U.S. can provide and what people really need.”

“We have repeatedly asked for specific support, and indeed, they are responding to that,” Edano said.

As the crisis worsened, the U.S. took measures to protect Americans in Japan, sending buses to pick up several hundred U.S. citizens who had been stranded north of Sendai, in the heart of the quake zone.

The Pentagon said Thursday that it had 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.

The U.S. government has instituted stricter guidelines for its citizens in Japan, urging people to stay at least 50 miles away from the plant — four times the distance suggested by Japanese officials. On Thursday, several other nations joined the U.S. in adopting the 50-mile recommended radius, including Canada, Britain and South Korea.

The diverging guidance fueled anxiety among some Japanese that they were not receiving reliable information about the emergency.

Speaking at the White House, a top energy official acknowledged that the situation is confusing. “The facts on the ground are genuinely complex,” said Deputy Energy Secretary Dan Poneman.

Greg Jaczko, chairman of the Nuclear Regulatory Commission, said the crisis at the Japanese facility “will likely take some time to work through, possibly weeks.”

On Thursday, the Japanese government raced to treat the damaged Fukushima Dai-ichi plant by land and air, trying to cool two reactor units in particular that have raised alarm about the prospect of a larger, imminent catastrophe.

In an emergency mission Thursday, two Japanese military helicopters dropped more than 30 tons of water on the plant. 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 Power said radiation levels showed a small decrease after the helicopter missions. The World Nuclear Association said the water drops by helicopter “did not appear accurate enough to be effective,” and that “the effect at present seems marginal at best.” The organization, which promotes nuclear energy, said one attempt was made to douse the unit 4 reactor building, but the pilots withdrew “after encountering high levels of radiation.”

Japan has made unit 3 a priority because government officials say the storage pool contains less water than that of unit 4. American officials, however, have said they believe unit 4’s pool could be empty of water, and the International Atomic Energy Agency later said unit 4 remains a major safety concern.