An official with the United Nations described the situation at Japan's crippled Fukushima Dai-ichi nuclear plant as "very serious."
Graham Andrew, a senior aide to International Atomic Energy Agency Director General Yukiya Amano, said the situation at the plant could change quickly, but "there had been no significant worsening" over the past 24 hours at the plant.
U.S. officials clearly see a much more dangerous situation than the Japanese regarding the nuclear plant. The top U.S. nuclear regulatory official warned of possible high emissions of radiation and the U .S. ambassador urged Americans within 50 miles (80 kilometers) of the plant to leave the area or at least remain indoors.
Chartered planes were being brought in by the U.S. government to help Americans leave. Thursday, Undersecretary of State Patrick Kennedy told a news conference that the first evacuation flight of U.S. citizens left Tokyo for Taipei, Taiwan. Kennedy said fewer than 100 people were onboard, mostly dependents of U.S. officials and also some other private citizens.
Kennedy said the U.S. would provide at least one other aircraft on Friday, depending on need.
Other countries are also advising citizens to leave Japan. The French government and Czech military have evacuated some of their citizens from Japan on special flights and Britain is planning the same.
Meantime, Tokyo Electric Power Co. paints a different picture compared with U.S. official accounts. The company, which owns the plant, said it believed workers were making headway in staving off a catastrophe with multiple cooling attempts.
Crews have tried to contain the situation with helicopter water drops on the complex's Unit 3. The helicopters dumped at least four loads on the reactor in just the first 10 minutes of the operation. Also, crews risked exposure to radiation as they flew in close to get the water onto the exposed radioactive material. They were being limited to 40 minutes in the area and dumped loads of about 2,000 gallons of water.
Along with the helicopter water drops, military vehicles designed to extinguish fires at plane crashes were also being used to spray the crippled Unit 3, said a military spokesman. The high-pressure sprayers were to allow emergency workers to get water into the damaged unit while staying safely back from areas deemed to have too much radiation.
But special police units trying to use water cannons - normally used to quell rioters - failed in their attempt to cool the unit when the water failed to reach its target from safe distances, said Yasuhiro Hashimoto, a spokesman for the Nuclear And Industrial Safety Agency.
Plant operators are also desperately trying to restore electricity to the plant which would allow them to use the onsite cooling system. However, U.S. officials fear the work is taking too long.
"It is my great hope that the information that we have is not accurate. I would hope for the sake of everyone that the situation is not in the state that we think it is," said Gregory Jaczko, chairman of the U.S. Nuclear Regulatory Commission.
According to ABC News, Japanese officials also have a plan to restore energy to restart the pumps used to cool the reactors. However, U.S. officials believe the pumps may not be functional even if energy is restored.
U.S. officials say they would like to see more workers at the plant. Currently, there are just 50 at any given time out of a total of about 180 workers.
The work involved in trying to cool the reactors is exposing the workers to dangerous levels of radiation. Japanese television has been sharing e-mails from loved ones expressing their concern. They are being hailed as heroes and nuclear crisis management experts say the workers are risking it all for their country and their people.
"I can tell you with 100 percent certainty they're absolutely committed to combating this casualty and doing whatever is humanly necessary to put these plants in safe condition," said Michael Friedlander, a former U.S. nuclear official.
The men do have an area at the plant that is safe from radiation. Experts say it is about the size of an average living room. It is there that they eat and sleep in shifts as the fight to contain this disaster continues.
Nuclear experts say the next 24 to 48 hours will be critical.
"This is very, very radioactive material. The winds can carry it great distances. If there is core on the floor and containment penetration, there will be significant public health consequences," said Nuclear safety expert, Dr. Kenneth Bergeron.
The phrase, "core on the floor" refers to a worst case scenario of the reactor containment walls being breached. If that happens, there will be no way to cool the core. It will melt down and send a plume of irradiated material into the air.
Nuclear Regularly Commission authorities have run future models based on current conditions and expectations, but ABC News reports that the range of possible scenarios is expansive due to the numerous unpredictable variables.
One thing that could make the situation worse would be a change in the weather. The winds have been pushing the irradiated material out to sea, but if they shift that could send the cloud toward populated areas.
All of this is happening while Japan copes with the aftermath of the massive magnitude-9.0 quake and tsunami.
The National Police Agency of Japan said 5,692 people are officially listed as dead, but authorities believe the casualty toll will climb to well over 10,000. About 9,522 were reported missing and hundreds of thousands have been left homeless. The agency also listed 2,409 people as injured.
The Associated Press contributed to this report.