With no water to cool the fuel rods, they could just get hotter and eventually melt down.
No water was left in the spent fuel pool of reactor 4, resulting in "extremely high" radiation levels.
Workers at the plant struggled to maintain water levels as the rods have heated up the water, threatening to evaporate it and expose the rods to air, which would send out radioactive material.
All of the water from one of the spent fuel-rod pools is gone, according to the chief of the U.S. Nuclear Regulatory Commission. With no water to cool the fuel rods, they could just get hotter and eventually melt down.
Gregory Jaczko, chair of the U.S. Nuclear Regulatory Commission, warned there was no water left in the spent fuel pool of reactor 4, resulting in "extremely high" radiation levels.
A police water cannon was deployed to help top up the water in the containment pool and expected to go into action early Thursday, Jiji Press news agency reported.
Workers at the plant, run by Tokyo Electric Power Co (TEPCO), have struggled to maintain water levels as the rods have heated up the water, threatening to evaporate it and expose the rods to air, which would send out radioactive material.
Helicopters which were to dump water on the plant were forced back by the radiation levels.
The U.S. military will send a spy drone to take a closer look at the reactors in the troubled plant, Kyodo News reported.
Engineers have been desperately battling a feared meltdown at the 40-year-old plant since the earthquake and tsunami knocked out cooling systems and fuel rods began overheating.
The 50 or so workers at the plant, which has been hit by four explosions and two fires, have been hailed as heroes.
"Please don't forget that there are people who are working to protect everyone's lives in exchange for their own lives," said one post on Japanese social networking site Mixi.
The US embassy in Tokyo early Thursday warned American citizens living within 50 miles (80 kilometers) of the plant to evacuate or seek shelter.
Switzerland has also advised its citizens to leave the quake-hit northeast and Tokyo, amid fears of further aftershocks and a widening nuclear disaster.
Japan's chief government spokesman Yukio Edano said radiation levels from the plant posed no immediate health threat outside the 20-kilometer (12-mile) exclusion zone.
But as crews scrambled to prevent a nuclear meltdown, the European Union's energy chief said the situation had spun out of control.
"The site is effectively out of control," energy commissioner Guenther Oettinger told a European Parliament committee, one day after he said Japan was facing "apocalypse."
France's Nuclear Safety Authority said the disaster now equated to a Level Six on the international scale for nuclear accidents, ranking the crisis second only in gravity to the level-seven Chernobyl disaster in 1986.
Yukiya Amano, the Japanese chief of the International Atomic Energy Agency, had insisted Tuesday there was no comparison to the Chernobyl crisis, when radiation spewed across Europe. He is due to visit Japan this week to get first-hand information on the situation.
Aside from the nuclear threat, the full scale of the quake and tsunami disaster was becoming clear as more details emerged of the staggering death and devastation in the worst-hit northeast.
"The number of people killed is increasing day by day and we do not know how many people have fallen victim," said the emperor, who is held in deep respect by many Japanese. "I pray for the safety of as many people as possible."
"People are being forced to evacuate in such severe conditions of bitter cold, with shortages of water and fuel... I cannot help praying that rescue work is done swiftly and people's lives get better, even a little."
And already jangled nerves were frayed further by a series of aftershocks including a strong 6.0-magnitude earthquake that swayed buildings in Tokyo.
The official toll of the dead and missing after the quake and tsunami flattened Japan's northeast coast rose to nearly 13,000, police said, with the number of confirmed dead at 4,314.
But reports continued to come in which indicated that the final grisly toll could be much higher, with the mayor of the coastal town of Ishinomaki saying the number of missing there was likely to hit 10,000, Kyodo News reported.
On Saturday, public broadcaster NHK reported that around 10,000 people were also unaccounted for in the port town of Minamisanriku, again in Miyagi prefecture.
Millions have been left without water, electricity, fuel or enough food and hundreds of thousands more are homeless, stoically coping with freezing cold and wet conditions in the northeast.
Aomori governor Shingo Mimura said he desperately needed central government assistance to get hold of oil and relief supplies.
"We cannot possibly get out to rescue survivors nor reconstruct the devastated areas without oil," he said.
"There are a variety of problems, such as shortages of water, food and blankets as well as difficulties in delivering supplies," added Ryu Matsumoto, state minister in charge of disaster management.
TEPCO said three-hour power outages Wednesday would affect 10.89 million households.
The governor of Fukushima prefecture, home to the crippled nuclear plant, said people were at breaking point.
"The worry and anger of the people of Fukushima has been pushed to the limit," Yuhei Sato told NHK.
With nerves on edge across the world's third-biggest economy and beyond, people across Asia have been stripping shelves of essentials for fear of a major emission of radiation from the power plant on the east coast.
The Japanese government has warned that panic buying in towns and cities that have not been directly affected by the twin disasters could hurt its ability to provide aid to the devastated areas.
The normally heaving streets and subways of Tokyo were quieter than usual on Wednesday. The number of people sporting paper face masks has shot up, although the masks offer no real protection against radiation.
Radiation levels in the capital's vast urban sprawl of 30 million people have see-sawed without ever reaching harmful levels, according to the government.
Beyond Japan, Asian nations vowed to crack down on hoax messages warning about radiation spreading beyond Japan, which have helped stoke growing unease over the unfolding crisis.