Updated at 2:06 p.m. Eastern
FUKUSHIMA, Japan - Emergency workers forced to retreat from a tsunami-stricken Japanese nuclear power plant when radiation levels soared prepared to return Wednesday night after emissions dropped to safer levels.
The pullback cost precious time in the fight to prevent a nuclear meltdown, further escalating a crisis spawned by last week's devastating earthquake and tsunami that pulverized Japan's northeastern coast and likely killed more than 10,000 people.
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Officials for Tokyo Electric Power Co., operator of the Fukushima Dai-ichi nuclear plant and also known as Tepco, said at a late night press conference Wednesday that four out of the five vehicles pumping seawater into the plant's reactors are operational. One pump vehicle each has been assigned to reactors No. 1 and No. 3. Reactor No. 2 has two pump vehicles. As of midnight Thursday, a new pump vehicle was being prepared to start working on reactor No. 4.
Meanwhile, the U.N.'s nuclear watchdog confirmed core damage in reactors Nos. 1, 2 and 3 in a news conference Wednesday. International Atomic Energy Agency chief Yukiya Amano called the situation in Japan "very serious" but said the plant's reactor vessels seem intact, the Reuters news agency reports. Amano said he hoped to leave Vienna for Japan Thursday and meet with senior officials.
It was unclear what happened in the nuclear plant's overheating reactors after late morning, when the workers stopped pumping in seawater trying to cool their fuel rods.
In Washington, Energy Secretary Steven Chu told members of a congressional committee that "we don't really know in detail what's happening," CBS News reports. Soon after Chu's testimony, the U.S. embassy in Tokyo urged all Americans within 50 miles of the power plant to evacuate the area or stay indoors. Reuters later reported that the British government advised its citizens in Tokyo and in areas north of the capital to consider leaving the area.
Ryohei Shiomi, of Japan's Nuclear and Industrial Safety Agency, told CBS News in a phone interview that the plant was never completely abandoned. He would not say how many workers remained inside the Fukushima Dai-ichi plant after the government suggested an evacuation was taking place.
Speaking to CBS Radio News' Lucy Craft earlier on Wednesday, a representative from Tepco said all the workers were back on the job.
But conditions at the Fukushima Dai-ichi nuclear plant appeared to be worsening. White steam-like clouds drifted up from one reactor which, the government said, likely emitted the burst of radiation that led to the workers' withdrawal. The plant's operator reported a fire at another reactor for the second time in two days.
"They are up against a wall ... this is not just one Three Mile Island. It's effectively four," Sharon Squassoni, an expert on nuclear issues at the Center for Strategic and International Studies, told CBS' "The Early Show."
At one point, national broadcaster NHK showed military helicopters lifting off to survey radiation levels above the complex, preparing to dump water onto the most troubled reactors in a desperate effort to cool them down. The defense ministry later said those flights were a drill, and it had no plans to make an airborne water drop.
Tepco officials said at their late night news conference that they wanted the military to again try pouring water from a helicopter onto the reactors, Reuters reported.
"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 already housing people moved from nearby the plant do not have enough hot meals and basic necessities.Radiation reality check: Risks and fears
Fears are so high, voluntary radiation exams are being administered as far as Niigata on the west coast of Japan. And Japanese nuclear authorities have a history of being less than forthcoming about the safety of their reactors, which only contributes to the confusion and distrust, reports CBS News correspondent Harry Smith.
The nuclear crisis has triggered international alarm and partly overshadowed the human tragedy caused by Friday's 9.0-magnitude earthquake and the subsequent tsunami, a blast of black seawater that pulverized Japan's northeastern coastline. The quake was one of the strongest recorded in history.
Millions of people struggled for a fifth day with little food, water or heat, and already chilly temperatures turned to snow in many areas. Police say more than 452,000 people are staying in temporary shelters, often sleeping on the floor in school gymnasiums.
Nearly 3,700 people are officially listed as dead, but officials believe the toll will climb over 10,000 since several thousand more are listed as missing.