The United States on Wednesday urged Americans who live within 50 miles of Japan’s earthquake-damaged Fukushima Daiichi nuclear power plant to evacuate, and the top U.S. nuclear regulatory official indicated that Japan faces an increasingly dangerous situation at one of the plant’s reactors.

Gregory Jaczko, chairman of the Nuclear Regulatory Commission, said Wednesday that no water remains in a deep pool used to cool spent fuel at the plant and that radiation levels there are thought to be “extremely high.”

Japanese officials denied that the water is gone from the spent-fuel pool, the Associated Press reported.

Jaczko, testifying before the House Energy and Commerce Committee on what his agency has been told about the crisis, said the plant’s unit 4 reactor appeared to have suffered a hydrogen explosion.

The reactor at unit 4 was shut down at the time of the earthquake last Friday, meaning that crews had transferred all of the radioactive fuel from the reactor’s core to the pool. The building housing the pool was damaged when two nearby reactor buildings exploded Saturday and Monday.

“It’s unprecedented,” said David Helwig, a retired nuclear engineer who spent 40 years working on boiling water nuclear reactors of the same design as those at Fukushima Daiichi. “That’s never happened before.”

Left exposed to the air, the fuel rods will start to decay and release radioactivity into the air.

Severe structural damage is the only way the fuel pool could be emptied, Helwig said. The 50-foot-deep pools have no outlets at the bottom, thus preventing them from draining in case of an accident.

The spent-fuel pool at another reactor, unit 3, also appeared compromised, Jaczko said.

The increasingly desperate picture of the struggle at the stricken nuclear plant emerged after Japanese helicopter crews abandoned an attempt to dump water on the pools of uranium fuel after detecting dangerous radiation above the plant.

Also Wednesday, the director general of the International Atomic Energy Agency confirmed damage to three of the facility’s six nuclear reactors, which contain radioactive fuel that is hotter, and more radioactive, than the fuel stored in the damaged pools.

In another sign of the worsening crisis, the U.S. ambassador to Japan warned Americans to stay at least 50 miles from the plant — more than four times the distance recommended in the Japanese government’s evacuation plan. Japanese authorities confirmed that crews at the plant had to temporarily abandon their posts as radiation readings spiked.

Jaczko’s testimony suggested that damage to the plant is worse than the Japanese government and the plant’s operator, Tokyo Electric Power Co., has acknowledged. On Tuesday, the company said water levels in the three of the site’s seven fuel pools were dropping, but did not say that the fuel rods themselves had been exposed.

Such exposure has been a huge concern among some longtime critics of the standard practice at nuclear facilities of storing used, or “spent,” fuel in structures that lack the heavy concrete-and-steel shields surrounding the reactor cores themselves.

A report on the Japanese crisis this week by Barclays Capital said, “Never, never, never allow the water level in a nuclear reactor to fall below the level of the fuel. This is the mantra pounded into the minds of nuclear power plant operators all over the world.” The report added, “It is hard to overemphasize the importance of the ‘keep the fuel covered’ training and design of these plants.” One of the report’s authors formerly provided such training at a U.S. commercial nuclear plant.

The only solution sounds simple but has apparently proved impossible for the Japanese crews: spray water by any means possible. “Just get a firehose up there if you can,” said Arnie Gundersen, a nuclear engineer.

The lack of water in at least one spent-fuel pool sparked fears of a worst-case scenario: the fuel could combust.

“If there’s no water in there, the spent fuel can star a fire,” said Eric Moore, a consultant to the Nuclear Regulatory Commission on nuclear plant design and safety issues. “Once you have that fire, there’s a high risk of radiation getting out, spewed by the fire.”

In Japan, authorities scrambled Wednesday to find ways to cool the overheated elements at the nuclear plant and prevent them from emitting potentially lethal radiation.

As radiation levels in the air above the plant spiked dangerously for the second consecutive day, U.S. Ambassador John V. Roos issued a recommendation based on a review of “the deteriorating situation” at the Fukushima Daiichi nuclear power plant by experts from the NRC and Energy Department.

“Consistent with the NRC guidelines that apply to such a situation in the United States, we are recommending, as a precaution, that American citizens who live within 50 miles (80 kilometers) of the Fukushima Nuclear Power Plant evacuate the area or to take shelter indoors if safe evacuation is not practical,” Roos said in a statement.

Japan has asked people living within 12 miles of the plant to evacuate and urged those living between 12 and 18 miles away to stay indoors. The Fukushima Daiichi plant, about 150 miles north of Tokyo, is one of Japan’s largest nuclear facilities and is normally capable of producing more than 4,500 megawatts of electricity.

The conflicting evacuation recommendations prompted a barrage of questions at Wednesday’s White House and State Department news briefings about the adequacy of the information Japan is providing to its citizens.

U.S. officials maintained that the 50-mile recommendation simply reflects the advice the NRC would give if such an incident occurred in the United States. White House spokesman Jay Carney said the recommendation is “based on new information and a deteriorating and fast-moving situation” and does not reflect adversely on “the quality of information or the level of cooperation” in Japan.

Earlier Wednesday, the Pentagon announced that U.S. forces participating in relief operations in Japan will not be allowed within 50 miles of the plant. Officials also said some flight crews are being issued potassium iodide tablets, which can reduce the risk of thyroid cancer from radiation exposure. The measure was described as precautionary. Several U.S. helicopter crews have been exposed to low levels of radiation, but no service members have shown signs of illness.

At Fukushima, a small group of workers charged with cooling efforts was temporarily relocated. Within an hour, though, the radiation levels dropped again, and the crew was permitted to return.

In order for them to resume trying to cool the damaged sectors, Japan’s health and welfare minister had to waive the nation’s standard of radiation exposure, increasing the level of acceptable exposure from 100 millisieverts to 250 — five times the level allowed in the United States.

The head of the International Atomic Energy Agency, Yukiya Amano, said in Vienna he plans to leave for Japan as soon as Thursday to get “firsthand information” on the situation at the stricken nuclear power plant. Amano, a former Japanese diplomat, said he wants to improve the flow of information to the IAEA about the crisis.

Workers at the Fukushima plant were focusing on the unit 3 reactor building, where a white plume of smoke was spotted Wednesday morning, and on unit 4, where fires flared up Tuesday and again on Wednesday morning.

The blazes triggered fears that spent uranium fuel sitting in a pool above the reactor was burning. Such a conflagration would generate intense concentrations of cesium-137 and other dangerous radioactive isotopes. But a spokesperson for the Nuclear Energy Institute, an industry lobbying group, said Tokyo Electric Power Co. concluded that the first fire in unit 4 was not in the spent fuel pool, “but rather in a corner of the reactor building’s fourth floor.”

Initially, government spokesman Yukio Edano said the steam coming from the unit 3 reactor building could mean that its containment vessel had ruptured in an earlier explosion — a potentially dire development. A reactor containment vessel in the plant’s unit 2 is believed to have ruptured on Tuesday.

But Edano said Wednesday afternoon that the unit 3 containment vessel was unlikely to have suffered severe damage. The Japanese news agency Kyodo quoted the country’s nuclear disaster task force as saying: “The possibility of the No. 3 reactor having suffered severe damage to its containment vessel is low.”

Still, Edano said officials presumed that the steam coming from unit 3 was indeed radioactive. He said emergency crews were still trying to determine its source .

 The rising steam was just the latest problem for the embattled plant, which suffered heavy damage to its cooling systems after Friday’s devastating earthquake and tsunami. Since then, the Japanese government and Tokyo Electric, which owns the facility, have struggled mightily to keep the plant’s six reactors cool. Each day has brought new problems.

Tuesday’s blast at unit 2 was not outwardly visible, but was potentially more dangerous than some of the earlier explosions, because it may have created an escape route for radioactive material bottled up inside the thick steel-and-concrete reactor vessel.

Radiation-laced steam is probably building between the reactor vessel and the building that houses it, experts said, creating pressure that could blow apart the structure, emitting radiation from the core.

“They’re putting water into the core and generating steam, and that steam has to go somewhere. It has to be carrying radiation,” said nuclear engineer Arnie Gundersen, who has 40 years of experience overseeing the Vermont Yankee nuclear facility, whose re­actors are of the same vintage and design as those at the Fukushima Daiichi plant.

Such a breach would be the first at a nuclear power plant since the Chernobyl catastrophe 25 years ago in what was then the Soviet Union.

Nuclear experts have repeatedly stressed that radiation releases on the scale of Chernobyl are unlikely or even impossible, given the Japanese plant’s heavier engineering and additional layers of containment. Still, Tokyo Electric said radiation briefly rose to dangerous levels at the plant Tuesday morning and again on Wednesday.

Crews noted a drop in pressure after the blast inside the unit 2 reactor and within a doughnut-shaped structure below, called a suppression pool. The simultaneous loss of pressure in those two places indicates serious damage, nuclear experts said.

The explosion probably happened after the streams of seawater that crews have been pumping into the reactor faltered. The fuel rods were left completely exposed to the air for some time, Tokyo Electric said in a statement. Without water, the rods grew white-hot and possibly melted through the steel-and-concrete tube.

The power company said a reduced crew of 50 to 70 employees — far fewer than the 1,400 or more at the plant during normal operations — had been working in shifts to keep seawater flowing to the three reactors now in trouble. Their withdrawal on Wednesday temporarily left the plant with nobody to continue cooling operations.

Wednesday afternoon, the Japanese military dispatched two helicopters to the Daiichi plant from Kasuminome Air Base in Sendai. A lead chopper was sent to determine whether radiation levels were low enough to continue with the operation. The second helicopter, a Boeing CH-47, followed behind, a huge bucket of sea water dangling beneath it. The CH-47 was slated to make several passes to drop water onto unit 3. But the crew on the first copter found radiation levels were too high to carry out the risky mission. 

Using a helicopter, or fire hoses, to spray water through holes in the breached buildings would be a risky, last-ditch effort to prevent the spent fuel from burning. With the outer containment building at unit 2 primed for a possible explosion, experts said, any fire crews would be in grave peril.

“This is scary,” said Lake Barrett, a nuclear engineer who directed the cleanup of the Three Mile Island nuclear facility in Pennsylvania after the 1979 accident there. “The plans in a severe accident are to just get a fire hose in there, get any kind of water to keep water in the pool above the fuel. ”

During normal plant operations, uranium fuel rods that can no longer produce enough heat for generating electricity are periodically removed from a reactor and placed into the spent-fuel pools above the reactors. These rods continue to generate heat and radioactive isotopes for many years.

Keeping this material covered with water is sufficient to cool it. But water levels may have dropped dramatically during the crisis, exposing fuel rods to the air.

Robert Alvarez, an analyst at the Institute for Policy Studies who has long warned of the dangers of spent fuel pools, said that — unlike the reactors themselves — the fuel pools typically do not have backup pumps to maintain water flow. “They were so overwhelmed,” he said of the workers straining to contain the disaster, that they were unable to maintain enough water in the pool to prevent boiling.

If the fuel pools are exposed to the air, the radiation doses coming from them could be life-threatening up to 50 yards, Alvarez said.

Concerns about the dangers of storing used uranium fuel in relatively poorly shielded pools above reactors increased with the fear of terrorism after the Sept. 11, 2001, attacks, causing industry experts to dispute the design. In 2006, the National Research Council issued a report saying in part that a uranium fuel fire “could result in the release of large amounts of radioactive material.”

The NRC report also recommended that nuclear power plants build “redundant and diverse” coolant systems to keep the fuel underwater during a crisis.

Late Tuesday, Tokyo Electric said water levels were “low” in unit 4’s used fuel pool. Japanese officials said Wednesday that the water level in unit 5 was slightly low but that they plan to use a generator to add more.

Satellite photos show steam rising from the facility. The amount of radioactivity carried by the plume is unknown, but small increases in radiation — not enough to affect human health — were reported in Tokyo and in other parts of Japan.

In response, NHK television reported that the Japanese government had ordered the country’s 47 prefectures to publicly report recorded radiation levels twice a day.

Wednesday morning, 33 nuclear disaster experts from the U.S. Department of Energy arrived in Japan with 17,000 pounds of gear to help with the crisis.

The teams will have plenty of work to do. The plant’s reactor cores take about two weeks to lose half of their intense heat, said Gundersen, the nuclear engineer, meaning that the battle between the radioactive cores and Fukushima Daiichi’s badly damaged cooling system will play out for days or weeks to come.

In Tokyo, Roos, the U.S. ambassador, stressed that Japan would continued to play the lead role in responding to the disaster.

“The Japanese government has significant expertise — and that’s probably an understatement saying ‘significant,’” Roos said. “They’re one of the most experienced countries in the world with regard to nuclear power and nuclear power plants.

“The United States government also has significant and massive expertise in the nuclear area . . . the United States has and will continue to provide any support it can in continuing to address the issues as they have arisen.”

Maese reported from Tokyo. Staff writers William Branigin and Debbi Wilgoren contributed to this report. Correspondent Akiko Yamamoto in Tokyo also contributed.