Tokyo (CNN) -- Technicians restored power to the control room of the No. 3 reactor at the quake-damaged Fukushima Daiichi nuclear power plant, the station's owner reported late Tuesday.

Tokyo Electric Power Company said it was able to confirm power by turning the control room's lights on. The next step, the company said, is to get air conditioning in the room so workers can enter and work there.

The No. 3 reactor has been a top priority for authorities trying to contain damage to the plant and stave off a possible meltdown. Its fuel includes a small percentage of plutonium mixed with the uranium in its fuel rods, which experts say could cause more harm than regular uranium fuels in the event of a meltdown.

Restoring power at the plant is a key step that officials hope will allow them to bring cooling systems back online.

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Earlier Tuesday, Tokyo Electric Vice President Sakae Muto said the No. 1 and No. 2 reactors at the plant suffered more damage from seawater than originally believed and will take more time to repair.

The tsunami that followed the 9.0-magnitude earthquake March 11 damaged electrical components and coolant pumps in units No. 1 and 2. Those are two of the three units now believed to have suffered damage to their reactor cores, Muto said.

Reactor No. 2 suffered more damage than No. 1, and the earliest those parts can be replaced is Wednesday, Muto said. The cause of the damage was unclear, but seawater was pumped in previously to cool the reactors as an emergency measure after the earthquake.

Reactors No. 3 and 4 were still being evaluated to determine which parts need repair or replacement, he said, adding that restoring lighting and air conditioning was a priority so crews can work from inside and gather further data.

Water was sprayed on the damaged housing of reactor No. 3 for about 50 minutes on Tuesday, and seawater was still being injected into the reactor core, Japan's Nuclear and Industrial Safety Agency said. Workers plan to spray over reactor No. 4 for three hours on Tuesday as well.

A faint trail of white smoke could be seen rising over the damaged nuclear plant earlier in the day. Japan's nuclear safety agency said it was not immediately clear why smoke was rising from the plant's No. 2 and No. 3 reactors. Tokyo Electric said in a statement that the smoke was decreasing and barely visible.

Officials temporarily evacuated workers from the plant Monday when smoke was spotted in the same area. But the agency said Tuesday that workers planned to continue efforts to restore power.

"This is going to be a two steps forward, one step back evolution," said Michael Friedlander, a former senior operator at three U.S. power plants who has been closely following the situation at the Fukushima Daiichi plant.

In terms of restoring power at the plant, workers are "on the verge of success," he told CNN's AC360.

But while work to reconnect cooling systems continued, workers and firefighters resumed spraying water at the buildings housing the No. 3 reactor Tuesday afternoon, Tokyo Electric said.

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Crews also began using concrete pumps to pour water on the No. 4 reactor building, Kyodo News reported.

Workers have been scrambling to cool down fuel rods since the earthquake and massive tsunami earlier this month knocked out cooling systems at the plant.

Police say the dual disaster has killed at least 9,080 people and left at least 13,561 missing, many of them killed as a wall of water rushed in following the quake.

Officials say some radiation has been released from the crippled plant since the quake and tsunami. And with the nuclear plant's six reactors in various states of disrepair, concerns have mounted over a potentially larger release of radioactive material from the facility, located about 240 kilometers (150 miles) north of Tokyo.

Officials said Tuesday that electrical cables had been laid to connect the No. 3 reactor and the neighboring No. 4 reactor with an outside power source.

That meant that power can now be funneled to all six of the plant's reactors for its cooling systems. But Tokyo Electric said Monday that electricity was still not moving to units No. 1 through No. 4 because the quake and tsunami had damaged numerous pumps and other gear. The company said spare parts were being brought in so that everything could work again.

Hidehiko Nishiyama of Japan's Nuclear and Industrial Agency said there was no evident explosion, spike in radiation or injuries at the No. 3 reactor Monday. The smoke came from the building's southeastern side, where the reactor's spent nuclear fuel pool is located, but the origin of the smoke at either reactor was unknown.

A spent fuel pool is also a concern in the building housing the No. 4 reactor.

The coolant pools contain used fuel rods that contain radioactive material and still generate high amounts of heat. Authorities have been working to keep them full to prevent the rods from being exposed and releasing radiation.

"The crisis has still not been resolved, and the situation at the Fukushima Daiichi nuclear power plant remains very serious," Yukiya Amano, the director-general of the International Atomic Energy Agency told the U.N. watchdog's board of governors Monday after a visit to the site.

On the other hand, Amano told reporters, rising pressure inside the containment unit at reactor No. 3, a concern from the weekend, was down and power had been restored to some of the reactors.

About 660 workers were at the site Tuesday, Tokyo Electric said. About 330 of those were employees of the power company.

Authorities said potential food contamination remains a concern.

The Japanese government has banned the sale of raw milk from Fukushima Prefecture, where the plant is located, and prohibited the sale of spinach from neighboring Ibaraki Prefecture after finding levels of radioactive iodine and cesium higher than government standards, the country's health ministry reported.

And officials in Fukushima halted the distribution of locally grown vegetables outside the prefecture.

High levels of radioactive substances have also been found in seawater near the plant, Tokyo Electric said Tuesday, adding that the results do not represent a threat to human health.

"There should be no immediate health impact. If this situation continues for a long period of time, some impact can occur," Chief Cabinet Secretary Yukio Edano told reporters.

Meltdown threat rises at Japanese nuclear plant

Reuters/Kyodo

A destroyed landscape in Otsuchi village, Iwate Prefecture in northern Japan. More photos »

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In this photo taken Sunday, March 13, 2011, the damaged Unit 1, left, and Unit 2, right, of the Fukushima Dai-ichi nuclear plant are seen in Okumamach

AP – In this photo taken Sunday, March 13, 2011, the damaged Unit 1, left, and Unit 2, right, of the Fukushima …

By ERIC TALMADGE and MARI YAMAGUCHI, Associated Press – 1 hr 11 mins ago

SOMA, Japan – Water levels dropped precipitously Monday inside a stricken Japanese nuclear reactor, twice leaving the uranium fuel rods completely exposed and raising the threat of a meltdown, hours after a hydrogen explosion tore through the building housing a different reactor.

Water levels were restored after the first decrease but the rods remained exposed late Monday night after the second episode, increasing the risk of the spread of radiation and the potential for an eventual meltdown.

The cascading troubles in the Fukushima Dai-ichi plant compounded the immense challenges faced by the Tokyo government, already struggling to send relief to hundreds of thousands of people along the country's quake- and tsunami-ravaged coast where at least 10,000 people are believed to have died.

Later, a top Japanese official said the fuel rods in all three of the most troubled nuclear reactors appeared to be melting.

Of all these troubles, the drop in water levels at Unit 2 had officials the most worried.

"Units 1 and 3 are at least somewhat stabilized for the time being," said Nuclear and Industrial Agency official Ryohei Shiomi "Unit 2 now requires all our effort and attention."

In some ways, the explosion at Unit 3 was not as dire as it might seem.

The blast actually lessened pressure building inside the troubled reactor, and officials said the all-important containment shell — thick concrete armor around the reactor — had not been damaged. In addition, officials said radiation levels remained within legal limits, though anyone left within 12 miles (20 kilometers) of the scene was ordered to remain indoors.

"We have no evidence of harmful radiation exposure," deputy Cabinet secretary Noriyuki Shikata told reporters.

On Saturday, a similar hydrogen blast destroyed the housing around the complex's Unit 1 reactor, leaving the shell intact but resulting in the mass evacuation of more than 185,000 people from the area.

Click image to see photos of quake, tsunami damage

AP/The Yomiuri Shimbun, Hiroto Sekiguchi

So the worst case scenario still hung over the complex, and officials were clearly struggling to keep ahead of the crisis.

Late Monday, the chief government spokesman said there were signs that the fuel rods were melting in all three reactors, all of which had lost their cooling systems in the wake of Friday's massive earthquake and tsunami

"Although we cannot directly check it, it's highly likely happening," Chief Cabinet Secretary Yukio Edano told reporters.

Some experts would consider that a partial meltdown. Others, though, reserve that term for times when nuclear fuel melts through a reactor's innermost chamber but not through the outer containment shell.

Officials held out the possibility that, too, may be happening.

"It's impossible to say whether there has or has not been damage" to the vessels, nuclear agency official Naoki Kumagai said.

If a complete reactor meltdown — where the uranium core melts through the outer containment shell — were to occur, a wave of radiation would be released, resulting in major, widespread health problems.

The Monday morning explosion at the Fukushima Dai-ichi plant's Unit 3 injured 11 workers and came as authorities were trying to use sea water to cool the complex's three reactors.

While four Japanese nuclear complexes were damaged in the wake of Friday's twin disasters, the Dai-ichi complex, which sits just off the Pacific coast and was badly hammered by the tsunami, has been the focus of most of the worries over Japan's deepening nuclear crisis. All three of the operational reactors at the complex now have faced severe troubles.

Operators knew the sea water flooding would cause a pressure buildup in the reactor containment vessels — and potentially lead to an explosion — but felt they had no choice if they wanted to avoid complete meltdowns. Eventually, hydrogen in the released steam mixed with oxygen in the atmosphere and set off the two blasts.

Japan's meteorological agency did report one good sign. It said the prevailing wind in the area of the stricken plant was heading east into the Pacific, which experts said would help carry away any radiation.

Across the region, though, many residents expressed fear over the situation.

People in the port town of Soma had rushed to higher ground after a tsunami warning Monday — a warning that turned out to be false alarm — and then felt the earth shake from the explosion at the Fukushima reactor 25 miles (40 kilometers) away. Authorities there ordered everyone to go indoors to guard against possible radiation contamination.

"It's like a horror movie," said 49-year-old Kyoko Nambu as she stood on a hillside overlooking her ruined hometown. "Our house is gone and now they are telling us to stay indoors.

"We can see the damage to our houses, but radiation? ... We have no idea what is happening. I am so scared."

Meanwhile, 17 U.S. military personnel involved in helicopter relief missions were found to have been exposed to low levels of radiation after the flew back from the devastated coast to the USS Ronald Reagan, an aircraft carrier about 100 miles (160 kilometers) offshore.

U.S. officials said the exposure level was roughly equal to one month's normal exposure to natural background radiation, and the 17 were declared contamination-free after scrubbing with soap and water.

As a precaution, the U.S. said the carrier and other 7th Fleet ships involved in relief efforts had shifted to another area.

While Japan has aggressively prepared for years for major earthquakes, reinforcing buildings and running drills, the impact of the tsunami — which came so quickly that not many people managed to flee to higher ground — was immense.

By Monday, officials were overwhelmed by the scale of the crisis, with millions of people facing a fourth night without electricity, water, food or heat in near-freezing temperatures.

International scientists say there are serious dangers but little risk of a catastrophe like the 1986 blast in Chernobyl, where there was no containment shells.

And, some analysts noted, the length of time since the nuclear crisis began indicates that the chemical reactions inside the reactor were not moving quickly toward a complete meltdown.

"We're now into the fourth day. Whatever is happening in that core is taking a long time to unfold," said Mark Hibbs, a senior associate at the nuclear policy program for the Carnegie Endowment for International Peace. "They've succeeded in prolonging the timeline of the accident sequence."

He noted, though, that Japanese officials appeared unable to figure out what was going on deep inside the reactor. In part, that was probably because of the damage done to the facility by the tsunami.

"The real question mark is what's going on inside the core," he said.

Overall, more than 1,500 people had been scanned for radiation exposure in the area, officials said.

___

Yamaguchi reported from Tokyo. Associated Press writers Tim Sullivan in Bangkok contributed to this report.