FUKUSHIMA, Japan – Nuclear plant operators trying to avoid complete reactor meltdowns said Thursday that they were close to completing a new power line that might end Japan's crisis, but several ominous signs have also emerged: a surge in radiation levels, unexplained white smoke and spent fuel rods that U.S. officials said could be on the verge of spewing radioactive material.

U.S. Nuclear Regulatory Commission Chairman Gregory Jaczko said in Washington on Wednesday that all the water was gone from the spent fuel pools at Unit 4 of the Fukushima Dai-ichi complex, but Japanese officials denied it. Hajime Motojuku, spokesman for plant operator Tokyo Electric Power Co., said the "condition is stable" at Unit 4.

If Jaczko is correct, it would mean there's nothing to stop the fuel rods from getting hotter and ultimately melting down. The outer shells of the rods could also ignite with enough force to propel the radioactive fuel inside over a wide area.

Jaczko did not say how the information was obtained, but the NRC and U.S. Department of Energy both have experts at the complex of six reactors along Japan's northeastern coast, which was ravaged by last week's magnitude-9.0 earthquake and subsequent tsunami.

The conditions at the plant appeared to worsen, with white smoke pouring from the complex and a surge in radiation levels forcing workers to retreat for hours Wednesday from their struggle to cool the overheating reactors.

As international concern mounted, the chief of the U.N. nuclear agency said he would go to Japan to assess what he called a "serious" situation and urged Tokyo to provide better information to his organization.

Japanese officials raised hopes of easing the crisis, saying early Thursday that they were close to completing a new power line that could restore the reactors' cooling systems.

Naoki Tsunoda, a spokesman for Tokyo Electric Power Co., or TEPCO, said the new power line to the Fukushima Dai-ichi plant was almost finished and that officials planned to try it "as soon as possible," but he could not say exactly when.

The new line could revive electric-powered pumps, allowing the company to maintain a steady water supply to troubled reactors and spent fuel storage ponds, keeping them cool. The company is also trying to repair its existing disabled power line.

Late Wednesday, government officials said they'd asked special police units to bring in water cannons — normally used to quell rioters — to spray water onto the spent fuel storage pool at Unit 4.

The cannons are thought to be strong enough to allow emergency workers to remain a safe distance from the complex while still able to get water into the pool, said Minoru Ogoda of Japan's nuclear safety agency.

TEPCO said it was also considering using military helicopters to douse the reactors with water, after giving up on such a plan because of high radiation levels in the atmosphere.

Wednesday's pullback by workers who have been pumping seawater into the reactors cost valuable time in the fight to prevent a nuclear meltdown, a nightmare scenario following the horrific earthquake and tsunami. The disasters last Friday pulverized Japan's northeastern coast and are feared to have killed more than 10,000 people.

The tsunami destroyed the complex's backup power system and left operators unable to properly cool nuclear fuel. The 180 emergency workers have been working in shifts to manually pump seawater into the reactors.

Japan's emperor, in an unprecedented made-for-TV speech, called on the country to work together.

"It is important that each of us shares the difficult days that lie ahead," said Akihito, 77. "I pray that we will all take care of each other and overcome this tragedy."

He also expressed his worries over the nuclear crisis, saying: "With the help of those involved I hope things will not get worse."

But officials are also taking increasing criticism for poor communication about efforts at the complex. There has been growing unease at the U.N.'s International Atomic Energy Agency's 35 board member nations, who have complained that information coming from Japan on the rapidly evolving nuclear disaster is too slow and vague.

IAEA head Yukiya Amano spoke of a "very serious" situation and said he would leave for Tokyo within a day.

He said it was "difficult to say" if events were out of control, but added, "I will certainly have contact with those people who are working there who tackled the accident, and I will be able to have firsthand information."

The nuclear crisis has partly overshadowed the human tragedy caused by Friday's 9.0-magnitude earthquake, one of the strongest recorded in history.

Millions of Japanese have been with little food and water in heavy snow and rain since Friday. In some towns, long lines of cars waited outside the few open gas stations, with others lined up at rice-vending machines.

National broadcaster NHK showed mammoth military helicopters lifting off Friday afternoon to survey radiation levels above the nuclear complex, preparing to dump water onto the most troubled reactors in an effort to cool them down.

The defense ministry later said those flights were a drill — then later said it had decided against making an airborne drop because of the high radiation levels.

"The anxiety and anger being felt by people in Fukushima have reached a boiling point," the governor of Fukushima prefecture, Yuhei Sato, fumed in an interview with NHK. He criticized preparations for an evacuation if conditions worsen, and said centers do not have enough hot meals and basic necessities.

More than 4,300 people are officially listed as dead, but officials believe the toll will climb to well over 10,000. Police say more than 452,000 people are staying in temporary shelters such as school gymnasiums.

Wednesday's radiation spike was believed to have come from the complex's Unit 3. But officials also acknowledged that they were far from sure what was going on at the four most troubled reactors, including Unit 3, in part because high radiation levels made it difficult to get very close.

While white smoke was seen rising Wednesday above Unit 3, officials could not ascertain the source. They said it could be spewing from the reactor's spent fuel pool — cooling tanks for used nuclear rods — or may have been from damage to the reactor's containment vessel, the protective shell of thick concrete.

Masahisa Otsuki, an official with TEPCO, said officials are most concerned about the spent fuel pools, which are not encased in protective shells.

"We haven't been able to get any of the latest data at any spent fuel pools. We don't have the latest water levels, temperatures, none of the latest information for any of the four reactors," he said.

In the city of Fukushima, meanwhile, about 40 miles (60 kilometers) inland from the nuclear complex, hundreds of harried government workers, police officers and others struggled to stay on top of the situation in a makeshift command center.

An entire floor of one of the prefecture's office buildings had been taken over by people tracking evacuations, power needs, death tolls and food supplies.

Elevated levels of radiation were detected well outside the 20-mile (30-kilometer) emergency area around the plants. In Ibaraki prefecture, just south of Fukushima, officials said radiation levels were about 300 times normal levels by late morning. It would take three years of constant exposure to these higher levels to raise a person's risk of cancer.

A little radiation was also detected in Tokyo, triggering panic buying of food and water.

Given the reported radiation levels, John Price, an Australian-based nuclear safety expert, said he saw few health risks for the general public so far. But he said he was surprised by how little information the Japanese were sharing.

"We don't know even the fundamentals of what's happening, what's wrong, what isn't working. We're all guessing," he said. "I would have thought they would put on a panel of experts every two hours."

Chief Cabinet Secretary Yukio Edano said the government expects to ask the U.S. military for help, though he did not elaborate. He said the government is still considering whether to accept offers of help from other countries.

There are six reactors at the plant. Units 1, 2 and 3, which were operating last week, shut down automatically when the quake hit. Since then, all three have been rocked by explosions. Compounding the problems, on Tuesday a fire broke out in Unit 4's fuel storage pond, an area where used nuclear fuel is kept cool, causing radioactivity to be released into the atmosphere.

Units 4, 5 and 6 were shut at the time of the quake, but even offline reactors have nuclear fuel — either inside the reactors or in storage ponds — that need to be kept cool.

Meanwhile, Japan's Nuclear and Industrial Safety Agency estimated that 70 percent of the rods have been damaged at the No. 1 reactor.

Japan's national news agency, Kyodo, said that 33 percent of the fuel rods at the No. 2 reactor were damaged and that the cores of both reactors were believed to have partially melted.

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Associated Press writers Elaine Kurtenbach and Shino Yuasa in Tokyo, David Stringer in Ofunato and Jocelyn Gecker in Bangkok contributed to this report.