After an explosion at one reactor at the Fukushima Dai-ichi nuclear complex left several people injured and prompted the evacuation of tens of thousands of people, authorities revealed a second reactor was in danger.
Chief Cabinet Secretary Yukio Edano said the atomic plant's Unit 3 was being pumped with seawater in an effort to prevent a hydrogen explosion and potential meltdown after its cooling system failed, AP reported.
"There is a possibility, we see the possibility of a meltdown," Japanese nuclear agency official Toshihiro Bannai told CNN. "At this point, we have still not confirmed that there is an actual meltdown, but there is a possibility."
More than 170,000 people have been evacuated as a precaution, according to Edano, who insisted that the small doses of radioactivity that have released into the atmosphere were no danger. However, Japan's nuclear safety agency said up to 160 people had been exposed to radiation.
The area around Fukushima Daini, a second nuclear plant in Japan's disaster-ravaged northeast, was hit by a 6.4-magnitude aftershock on Saturday, an echo of the powerful seismic event that unleashed destructive waves onto the area's coast on Friday.
On Sunday, authorities said the death toll from the twin disaster is expected to number well into the thousands, with police in the badly-affected Miyagi prefecture estimating at least 10,000 had been killed there alone.
Japan's meteorological agency on Sunday revised the quake's magnitude up to 9.0, making it the fourth most powerful ever recorded, the Kyodo news agency said. Scientists say the event's power tilted the Earth's axis by four inches.
As a massive rescue and relief operation continued, new details of the scale of the disaster were emerging. Aerial photos showed the entire coastal resort town of Minami Sanriku — 55 miles from the quake's epicenter — barely visible under floodwaters.
Newly-released footage showed the wave hitting the port town of Miyako, pulverizing fishing boats and surging over sea defenses to swallow neighborhoods and roads. Cars were swept effortlessly from parking lots as residents watched in horror.
As the threat from the atomic plants added to Japan's woes, thousands of families were forced to seek emergency shelter after fleeing their homes under threat of radioactive contamination.
"Families lay huddled together for warmth under the blankets, children between grandparents and their brothers and sister," the Daily Telegraph reported from the town of Iwaki, 18 miles from the nuclear plants.
"Others clustered around paraffin heaters, although they were quick to extinguish them whenever an aftershock struck. When new shocks came — as they did, frequently — the light fittings rattled over their heads."
Reiko Takagi, another Iwaki citizen, told the Guardian she wanted to flee the area but was struggling because of quake damage.
"Everyone wants to get out of the town. But the roads are terrible," she said. "It is too dangerous to go anywhere. But we are afraid that winds may change and bring radiation toward us."
Edano said: "There is no confusion at this point, although we appreciate that people will have to leave their homes and livelihoods behind, but there is no panic."
With the situation at the nuclear plants still uncertain, there was debate about how serious the threat of radiation was and whether Japan could be facing a third disaster similar to the Chernobyl explosion 25 years ago.
The International Atomic Energy Agency has been issuing weather forecasts for the affected areas to anticipate the possible spread of nuclear fallout, but some experts say the danger is likely to mitigated by Japanese power station design.
"Yaroslov Shtrombakh, a Russian nuclear expert, said it was unlikely that the Japanese plant would suffer a meltdown like the one in 1986 at Chernobyl," AP reported. "That reactor, unlike the reactor at Fukushima, was not housed in a sealed container."