Nuclear plant engineers in Japan are frantically working to keep temperatures down in a series of reactors, as the possibility of a nuclear disaster threatens to complicate matters following a devastating earthquake and tsunami.
Authorities declared a state of emergency at a nuclear facility in the town of Onagawa on Sunday after excessive radiation levels were recorded there, the UN's International Atomic Energy Agency said.
Engineers have faced fears of a partial meltdown in two reactors at the Fukushima Dai-ichi nuclear plant in the wake of Friday's earthquake and tsunami that are believed to have killed thousands of people.
Yukio Edano, Japan's chief cabinet secretary, said earlier on Sunday that a partial meltdown in Unit 3 at the Fukushima facility was "highly possible".
"At the risk of raising further public concern, we cannot rule out the possibility of an explosion," Edano said. "If there is an explosion, however, there would be no significant impact on human health.
"Because it's inside the reactor, we cannot directly check it but we are taking measures on the assumption of the possible partial meltdown."
About 170,000 people have been ordered to evacuate the area covering a radius of 20km around the plant.
Thousands of people have been taken to emergency shelters along the northeastern coast as strong aftershocks continue to shake Japan's main island.
Naoto Kan, the Japanese prime minister, said the current situation is the worst disaster the country has faced since the second world war - in which more than 200,000 people were killed in the 1945 atomic bombings of Hiroshima and Nagasaki by the US.
"As to Friday's earthquake and tsunami and the current situation of the power plants in Fukushima, in the 65 years after the end of World War II, this is the toughest and the most difficult crisis for Japan in that period," Kan said in a televised address to the nation.
He also said Japan is at risk of large-scale power outages and must save energy after the earthquake shut down some atomic power plants.
Plea for understanding
The power supply situation in affected areas was "extremely severe", Kan said and appealed for people's understanding of the need for phased, scheduled power cuts to avoid unscheduled power cuts which could cause enormous damage.
The quake and tsunami damaged three reactors at the Fukushima Dai-ichi nuclear plant, which lost their cooling functions necessary to keep the fuel rods working properly.
Ilham al-Qaradawi, a professor of nuclear physics at Qatar University, told Al Jazeera that even if a meltdown occurs, it might not necessarily become dangerous.
New footage shows the moment the massive tsunami slammed into Japan's coastline
"It depends on how this is going to be contained by the containment of the reactor," she said.
"It could be that the reactor core would be completely damaged but there is no releases of radioactivity and that's the important part.
"Chernobyl is a much worse situation than this case. Despite the slight release of radiation I think these reactors have proven to be very good reactors ... so far in terms of radiation, contamination has not been very pronounced."
Operators released slightly radioactive air from Unit 3 on Sunday, while injecting water into it as an effort to reduce pressure and temperature to save the reactor from a possible meltdown, Edano said.
A meltdown refers to a very serious collapse of a power plant's systems and its ability to manage temperatures.
A complete meltdown would release uranium and dangerous byproducts into the environment that can pose serious health risks.
The warning from Edano, the chief cabinet secretary, of the risk of a second explosion at the Fukushima plant came a day after a blast destroyed the building housing another reactor.
He said radiation levels briefly rose above legal limits, but that had since declined significantly.
Cause of blast
On Saturday, an explosion destroyed the building housing the Unit 1 reactor but not the reactor itself, which is enveloped by stainless steel 15cm thick.
The cause of the blast was apparently a reaction of hydrogen and oxygen as some hydrogen gas was released to relieve pressure inside the reactor.
Officials have said radiation levels at Fukushima were elevated before the blast. At one point, the plant was releasing each hour the amount of radiation a person normally absorbs from the environment each year.
Workers in protective clothing were scanning people arriving at evacuation centres for radioactive exposure.
Officials said 22 people were known to have been exposed to radiation.
Virtually any increase in dispersed radiation can raise the risk of cancer, and authorities were planning to distribute iodine, which helps protect against thyroid cancer.