As Japan struggled to tame a quake-damaged nuclear power plant, a new fire was reported early Wednesday in a building housing one of the plant’s reactors, the latest setback following a series of hydrogen explosions and leaks of radioactivity.

The Tokyo Electric Power Co., which operates the stricken Fukushima Daiichi plant about 150 miles north of Tokyo, said a worker saw a fire at about 5:45 a.m. Wednesday in the northeast corner of the building that contains the unit 4 reactor.

The company said firefighters were trying to extinguish the blaze, whose cause was not immediately known.

A fire burned for two hours Tuesday in the pool containing spent nuclear fuel rods at the unit 4 reactor. News services quoted a government official as saying the pool could still be near the boiling point.

The latest blow to an increasingly beleaguered and frightened populace came after Japanese officials, racing to avert a potential nuclear catastrophe, said Tuesday that they were considering a risky plan to spray water from a helicopter to prevent new fires in a pool of spent fuel at the Fukushima Daiichi power plant.

All but about 50 workers were evacuated from the plant, where at least three reactor cores are believed to be imperiled, and Prime Minister Naoto Kan hailed those who remained, saying they “are putting themselves in a very dangerous situation.”

Explosions destroyed the tops of two buildings housing reactors at the plant, one on Saturday and another on Monday. An explosion Tuesday in another reactor, unit 2, “may have affected the integrity of the primary containment vessel,” the International Atomic Energy Agency reported.

In addition to the helicopter operation, Tokyo Electric will also try to spray water from trucks on the ground through a hole in the building around the pool, NHK television reported.

Such drastic measures would be a last-ditch effort to prevent the spent fuel from burning and to keep cesium-137 and other radioactive isotopes from being released into the air, experts said.

“This is scary,” said Luke Barrett, a nuclear engineer who directed the clean-up of the Three Mile Island nuclear facility in Pennsylvania. “The plans in a severe accident are to just get a firehose in there, get any kind of water to keep water in the pool above the fuel. To try to lower buckets of water through the roof with a helicopter is a huge challenge, especially for the pilots.”

With the outer containment building at unit 2 primed for a possible explosion, any helicopter crews hovering over nearby unit 4 would be in grave peril, Barrett said.

During normal plant operations, uranium fuel that can no longer produce enough heat for generating electricity is periodically removed from a reactor and placed into the spent fuel pools located above the reactors. The spent fuel continues 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 in the pools at reactor 4. Exposed fuel can easily burn, which is believed to be what happened during the fire in the fuel pool Tuesday.

Robert Alvarez, an analyst at the Institute for Policy Studies who has long warned of the dangers of spent fuel pools at reactors, 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, “maybe they weren’t checking water levels, and water started heating up and boiling” in the pool.

If the fuel pools are exposed to the air, the radiation doses coming from them could be life-threatening within 25 to 50 yards, Alvarez said.

Satellite photos taken Monday show steam rising from the damaged unit 3 building. The amount of radioactivity carried by the plume is unknown, but small increases in radiation — not enough to impact human health — were reported Tuesday in Tokyo, about 150 miles to the southwest of the stricken facility.

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.

Officials from Tokyo Electric said radioactive substances were emitted after the explosion in the unit 2 reactor at 6:14 a.m. (5:14 p.m. Monday in Washington). The blast took place near or in the suppression pool, which traps and cools radioactive elements from the containment vessel, officials said. The explosion appeared to have damaged valves and pipes, possibly creating a path for radioactive materials to escape.

While the fire at the fourth reactor had been extinguished, Japanese officials told the IAEA that because of the blaze “radioactivity is being released directly into the atmosphere,” the agency said.

Kan, looking grave, told the nation that radiation already had spread from the reactors and there was “still a very high risk of further radioactive material escaping.” He urged people within 12.5 miles to evacuate the area, and said those within 19 miles of the plant should remain indoors.

Roughly nine hours after the explosion, radiation levels just outside the plant — at dangerous levels in the morning — had fallen to 496 micro sieverts per hour, a concentration “much higher than the normal level ... but one that causes no harm to human health,” government spokesman Yukio Edano said. At 9 a.m. (8 p.m. Monday in Washington), the radiation level at the plant had been 11,930 micro sieverts per hour, several times the amount a human should receive in one year.

Speaking about the dropping radiation levels, Edano said, “In that sense, we are relieved.”

Higher-than-normal radiation levels were detected in Tokyo, roughly 150 miles from Fukushima. Kanagawa, a prefecture south of Tokyo, recorded radiation at nine times the usual level. In Ibaraki, roughly 70 miles from Tokyo, levels were briefly 100 times the normal measure, according to the Kyodo news agency.

In each case, officials said that exposure to those levels of radiation would not pose an immediate danger to human health.

But many residents said they were deeply worried, and scores of foreign residents of Japan made plans to leave the country as soon as possible.

A no-fly zone was declared covering a 19-mile radius around the Fukushima Daiichi facility. For most of the day, winds blew in a southeasterly direction, pushing the plume of radioactivity toward the Pacific Ocean.

Late Tuesday afternoon in Japan, authorities said the situation at Fukushima Daiichi had marginally improved — though it remains dangerous. In addition to putting out the fire at unit 4, workers were closer to stabilizing units 1 and 3, keeping the fuel rods under the necessary cooling water. Edano said that it was too early to tell if workers’ emergency cooling efforts are working for unit 2.

“There is no manual to this kind of incident. I believe on the ground things are chaotic,” Takayuki Terai, professor of nuclear engineering at the University of Tokyo. “But in essence, they just have to put water into the reactors continuously and cool them down and contain them.”

Amid the four-day-long emergency at the Fukushima Daiichi plant, Japan’s public has grown skeptical about the reliability of official information, criticizing Tokyo Electric officials in particular for their vague answers during news conferences.

Kan himself was not briefed on the Tuesday morning explosion until after it had been reported on television. According to a Kyodo reporter who overheard the conversation, Kan later grilled the company representatives, asking, “What the hell is going on?”

During a midmorning news conference, four Tokyo Electric officials revealed almost no information about the blast.

Japan’s usually deferential news media turned vicious, asking, “What does this mean?”

“We want answers, not apologies,” one reporter said.

Tuesday began with the fire that broke out in a pool storing spent fuel rods at the base of unit 4, which had been shut down for inspection before last Friday’s earthquake. Experts said the fire most likely broke out because the pool water had run low or dry, allowing the rods to overheat. Radioactive substances spewed outside from the fire, officials said, because the structure housing the pool was damaged by Monday’s explosion at unit 3.

Half an hour later, the explosion at unit 2 took place. Experts said that, unlike the two previous explosions that destroyed outer buildings, this explosion might have damaged portions of the containment vessel designed to bottle up radioactive materials in the event of an emergency.

The explosion was followed by a brief drop in pressure in the vessel and a spike in radioactivity outside the reactor to levels more than eight times the recommended limit for what people should receive in a year, the company said. Japanese government officials later said it was unclear whether the spent fuel fire or the explosion had caused the spike in radiation.

The new setbacks came on the heels of a difficult Monday at Fukushima Daiichi unit 2, one of six reactors at the site. Utility officials there reported that four out of five water pumps being used to flood the reactor had failed and that the other pump had briefly stopped working. As a result, the company said, the fuel rods, normally covered by water, were completely exposed for 140 minutes.

That could have grave consequences, worsening the partial meltdown that most experts think is underway. By comparison, in the 1979 Three Mile Island nuclear plant accident, it took two hours for half the plant’s nuclear fuel to melt.

According to a report by the Kyodo news agency, the fifth pump was later restarted, and seawater mixed with boron was again injected in a desperate bid to cool the reactor, but the fuel rods remained partially exposed and ultra-hot. On Tuesday morning, Tokyo Electric said that 2.7 meters (3 yards), or less than half, of the rods were still exposed.

The other four pumps were thought to have been damaged by a blast Monday that destroyed a building at the nearby unit 3 reactor, Kyodo reported. That blast, like one on Saturday at unit 1, was caused by a buildup in hydrogen generated by a reaction that took place when the zirconium alloy wrapped around the fuel rods was exposed to steam at 2,200 degrees Fahrenheit.

The International Atomic Energy Agency reported that injections of seawater into units 1 and 3 had been interrupted because of a low level in a seawater supply reservoir, but the seawater injections were later restored.

A commercial satellite photo of the complex showed piles of debris on top of units 1 and 3, which raised new fears about the condition of the pools where spent fuel is stored, especially at unit 1, where a design by General Electric placed the pool on top of the reactor but below the outer structure that was destroyed. In the satellite photo, there was no sign of a large crane that had been sitting on the roof before the blast. The ability of workers to assess the damage was hindered by fears that another explosion might occur.

In March 2010, 1,760 tons of spent fuel was stored in the six pools — 84 percent of capacity, according to Tokyo Electric.

The string of earthquake- and tsunami-triggered troubles at the Fukushima Daiichi plant began Friday, when a loss of grid power (caused by the earthquake) followed by a loss of backup diesel generators (caused by the tsunami) led to the failure of cooling systems needed to keep reactor cores from overheating.

The IAEA reported that Japan has evacuated 185,000 people from towns near the nuclear complex. The agency said Japan has distributed 230,000 units of stable iodine to evacuation centers from the area around the Fukushima Daiichi and Fukushima Daini nuclear power plants. The iodine has not been administered to residents; the distribution is a precautionary measure. The ingestion of stable iodine can help to prevent the accumulation of radioactive iodine in the thyroid.

As the crisis in Japan continued into its fifth day, many nuclear experts called for a tougher scrutiny of U.S. plants, noting that Japanese’s problems were exposing the limits of human ingenuity and imagination and pointed to the possible failure of the best-laid backup plans.

David Lochbaum, a nuclear engineer and director of the Nuclear Safety Program of the Union of Concerned Scientists, said in a conference call that in certain respects, the U.S. nuclear plants are not as prepared as the Japanese ones for a catastrophic power outage. After the earthquake and tsunami knocked out the electrical grid and backup generators, the Japanese engineers switched to batteries that could last for eight hours, he said.

“In this country, most of our reactors are only designed with battery capacity for four hours,” Lochbaum said. “Many of our reactors are in situation where earthquakes, or hurricanes in the gulf, or ice storms in the northeast, or a tree in Cleveland, can cause an extensive blackout,” he said.

The August 2003 blackout in North America that affected 52 million people across the upper Midwest, New York and parts of Canada was triggered when overheated wires sagged into trees in northeastern Ohio. Nine nuclear units switched to diesel backup generators, which are the size of locomotives without wheels.

Harlan reported from Tokyo. Staff writers William Branigin, Joel Achenbach and Steven Mufson in Washington and correspondent Akiko Yamamoto in Tokyo contributed to this report.