Japan battled a feared meltdown of two reactors at a quake-hit nuclear plant Sunday, as the full horror of the disaster emerged on the ravaged northeast coast where more than 10,000 were feared dead.
An explosion at the ageing Fukushima No. 1 atomic plant blew apart the building housing one of its reactors Saturday, a day after the biggest quake ever recorded in Japan unleashed a monster 10-metre (33-foot) tsunami.
The atomic emergency widened Sunday as the cooling systems vital for preventing overheating failed at a second reactor, and the government warned there was a risk it too could be hit with a blast.
"There is the possibility of an explosion in the number-three reactor," said Yukio Edano, the top government spokesman, while voicing confidence it would withstand the blast as the number-one reactor had the day before.
Edano, the chief cabinet secretary, said earlier it was highly likely a meltdown had occurred in the first reactor, at the plant situated on the coast 250 kilometres (160 miles) northeast of Tokyo.
"As for the number-three reactor, we are acting on the assumption that it is possible," he said.
Edano said some radiation had escaped, but that the levels released into the air had so far not reached levels high enough to affect human health.
The colossal 8.9-magnitude tremor sent waves of mud and debris racing over towns and farming land in Japan's northeast, destroying all before it and leaving the coast a swampy wasteland.
In the small port town of Minamisanriku alone some 10,000 people were unaccounted for -- more than half the population of the town, which was practically erased, public broadcaster NHK reported.
The police chief in Miyagi prefecture -- where Minamisanriku is situated -- said the death toll was certain to exceed 10,000 in his district alone.
As the world's third-largest economy struggled to assess the full extent of what Prime Minister Naoto Kan called an "unprecedented national disaster", groups of hundreds of bodies were being found along the shattered coastline.
"We have received a preliminary report that more than 200 bodies were found in the city of Higashimatsushima," a National Police Agency spokesman said in the latest find on Sunday.
Edano said at least 1,000 people were believed to have lost their lives, and police said more than 215,000 people were huddled in emergency shelters.
In the city of Fukushima, about 80 kilometres (50 miles) northwest of the stricken plant, AFP reporters saw panic buying at supermarkets and said petrol stations had run dry.
In Minamisoma town, which was virtually obliterated by the tsunami's black tide of mud and debris, an AFP reporter saw fire volunteers collecting bodies found in the twisted wreckage of what had once been a residential area.
An elderly woman wrapped in a blanket tearfully recalled how she and her husband were evacuated from Kesennuma town, another fishing port which the tsunami swept through.
"I was trying to escape with my husband, but water quickly emerged against us and forced us to run up to the second story of a house of people we don't even know at all," she told NHK.
"Water still came up to the second floor, and before our eyes, the house's owner and his daughter were flushed away. We couldn't do anything. Nothing."
The sheer power of the water tossed cars like small toys, upturned lorries that now litter the roads and left shipping containers piled up along the shore.
In Sendai city, where the haunting drone of tsunami sirens had echoed into the night, a hospital used generators to keep its lights blazing, drawing in wearied survivors, but supplies of food and fuel were fast running out.
"We have asked other hospitals to provide food for us, but transportation itself seems difficult," Sendai Teishin Hospital spokesman Masayoshi Yamamoto told AFP.
The International Atomic Energy Agency said about 200,000 people had so far been evacuated from the area around the two Fukushima plants that house a total of 10 reactors.
Japan's nuclear safety agency rated the incident at four on the international scale of zero to seven. The 1979 Three Mile Island accident in the United States was rated five, while the 1986 Chernobyl disaster was a seven.
After Saturday's blast, which sent smoke billowing into the sky, the government moved to calm growing fears, saying the explosion did not rupture the container surrounding the reactor itself.
Workers doused the stricken No.1 reactor with sea water to try to avert catastrophe, in what US experts warned was an "act of desperation" that, in the worst-case scenario, could foreshadow a much more serious disaster.
The plant's operator said that so much water had evaporated from the number-three reactor that at one stage the top three metres (10 feet) of the fuel rods were exposed to the air, although they were later covered again.
Japan's ambassador to the United States Ichiro Fujisaki told CNN: "There was a partial melt of a fuel rod, melting of fuel rod. There was a part of that... but it was nothing like a whole reactor melting down."
A total of 22 people have been hospitalised after being exposed to radioactivity, although it was not immediately clear to what degree they were exposed and what condition they were in.
Japan committed 100,000 troops -- about 40 percent of the armed forces -- to spearhead a mammoth rescue and recovery effort with hundreds of ships, aircraft and vehicles headed to the Pacific coast area.
"There are so many people who are still isolated and waiting for assistance. This reality is very stark," Defence Minister Toshimi Kitazawa was quoted as saying by Kyodo News.
The world rallied behind the disaster-stricken nation, with offers of help even from Japan's traditional rivals. Despite a territorial row that has soured relations, China sent a team of rescuers who were due to arrive Sunday.
The US aircraft carrier Ronald Reagan reached waters off the northeast coast Sunday, part of a flotilla sent by Japan's close ally which has nearly 50,000 military personnel in the country.
The massive earthquake, one of the largest in recorded history, appears to have shifted the main Japanese island by about eight feet (2.4 meters), the US Geological Survey said.
Two days after it struck about 400 kilometres northeast of Tokyo, aftershocks were still rattling the region, including a strong 6.8 magnitude tremor on Saturday and a 6.3 quake on Sunday.
Japan sits on the "Pacific Ring of Fire", and Tokyo is in one of its most dangerous areas, where three continental plates are slowly grinding against each other, building up enormous seismic pressure.