It is "highly possible" that a partial meltdown was occurring in one of the nuclear reactors damaged in Friday's powerful earthquake, a Japanese government spokesman said today, the most dire statement yet of the situation at the power plant.
Measures were taken at Unit 3 of the Fukushima Daiichi power plant today, including releasing radioactive air and injecting sea water to reduce pressure and cool the reactor down, to prevent a possible meltdown, Chief Cabinet Secretary Yukio Edano said.
But he also said it is "highly possible" there has been a partial meltdown in the unit.
"Because it's inside the reactor, we cannot directly check it but we are taking measures on the assumption of the possible partial meltdown," he said, according to The Associated Press.
The government had earlier denied that there was any possible meltdown.
Today, according to the AP, Edano said the radiation around the reactor rose briefly above legal limits, but has since declined significantly, and he said the rods in the reactor were briefly exposed.
In all, cooling systems have failed at six of the reactors at two Fukushima nuclear plants, including the reactor that exploded, where observers and experts have also feared a meltdown could occur.
The extent of the damage a the reactors and the cause of the explosion, which led Japanese officials to order tens of thousands to evacuate the region around the plant, are not clear.
According to Japan's Nuclear and Industrial Safety Agency -- an independent body -- the only thing that could have caused the explosion was a meltdown of the reactor core.
Along with the uncertainty about the nuclear facilities, Japan continues to be shaken by aftershocks from the 8.9 magnitude earthquake that hit Friday.
The latest temblor, which had a magnitude of 6.2 according to the U.S. Geological Survey, was centered off the eastern coast, but closer to Tokyo than the quake that devastated parts of the country and triggered a tsunami that hit the west coast of North America.
The official death toll from Friday's earthquake and tsunami rose to 763, while local media reports put fatality totals closer to 1,300 people. With thousands unaccounted for in the hardest hit areas, that number is expected to rise.
Police said between 200 and 300 bodies were found along the coast in Sendai, the largest city in the area near the quake's epicenter.
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The explosion early Saturday ripped through one of the buildings on the Fukushima Daiichi plant, injuring four workers.
Radiation levels were high before the explosion -- at one point releasing as much radiation every hour as a person would normally absorb from the environment in a year -- but radiation outside the plant started decreasing after the blast and the pressure inside the reactor was also dropping, the spokesman said.
A government spokesman said the blast did not damage the nuclear reactor itself, which would cause radioactive material to leak out.
But Ryohei Shiomi with Japan's Nuclear Safety Commission said a meltdown was possible.
The evacuation radius has now been expanded to 20 kilometers (12 miles).
The majority of the 51,000 people living near the danger zone have already been evacuated, according to Shiomi.
Three evacuees have been exposed to radiation, but have not shown signs of illness, a disaster official told The Associated Press.
Japanese authorities say they have plans to distribute iodine to residents in the area around both the Fukushima Daiichi and nearby Fukushima Daini plants.
Search and Rescue Continues
Meanwhile, Japanese authorities are racing to rescue those trapped in the rubble after an8.9 magnitude earthquake and a tsunami left hundreds dead and a nuclear reactor on the verge of a possible of meltdown.
Prime Minister Naoto Kan said he dispatched 50,000 troops for recovery efforts as powerful aftershocks continue to rattle the region.
Tsunami survivors were plucked by helicopters and from rooftops, but hundreds more along the 1,300-mile stretch of coastline are waiting to be rescued. There are 200,000 people living in temporary shelters after being evacuated to higher ground and more than 1 million households are without water. Five million households lack electricity.
The earthquake, the fifth largest in recorded history and the largest ever to hit Japan, struck about 2:46 p.m. local time.
It triggered a tsunami that unleashed a menacing stew of debris with objects as large as ships, cars and houses coursing over the countryside and into towns, crushing buildings and everything in its path. Eerily, fires burned in the watery mess as it flowed along.
Entire villages were swept away and four bullet trains with an unknown number of people onboard lost contact with rail operators Friday, according to the Kyodo News Agency.
Hours after the buildings stopped shaking, fires still burned and Tokyo remained largely paralyzed with phone and train service halted on Friday. Four million buildings were without power.
Many in Tokyo spent the night in their cars trying to get home because the highways were closed and cars clogged the city's streets.
Store shelves were stripped bare by shoppers and thousands more spent the night in their offices.
It was harder to assess damage outside the capital because of cut phone communications.
Much of the town of Kesennuma near Miyagi, burned during the night with no apparent hope of being extinguished, public broadcaster NHK said.
Japan's coast guard was searching for 80 dock workers on a ship that was swept away from a shipyard in Miyagi.
U.S. Mobilizes Humanitarian, Military Aid
President Obama called the tsunami "catastrophic" and said help was coming. The United States has one aircraft carrier in Japan and another is on the way, and a ship is also en route to Marianas Islands for assistance, Obama said.
A USAID Disaster Urban Search and Rescue teams of more than 150 people are headed to Japan to assist in the rescue effort. The Pentagon is sending some P-3 Orion maritime surveillance aircrafts to support the Japanese government by providing aerial reconnaissance over quake-damaged areas.
So far there are no reports of American fatalities in Japan, but anxious posts on Facebook by Americans living there reveal the devastation in the hard-hit Sendai area.
Randy Castle, an American working in Japan, said his hotel lobby was full of people overnight who couldn't make it home from work.
"It started and it lasted a good five minutes, lots of shaking and very scary. I'm on the 11th floor, just down the street from the Tokyo tower," Castle said.
The Tokyo tower, a famed landmark in Japan, now stands bent.
"It was a lot of swaying, you could hear the building creaking in it ... you could see the shades shaking back and forth," Castle said. "The people that I work with here in Tokyo, it was normal for them, but shortly after that it started to get scary the longer it went."
"We were just hanging out in Shibuya [Tokyo] today and walking down the street and all of a sudden felt like we were on a boat and looked up and tall buildings were going crazy, looking like they were going to tip over," said Kevin Williams, an American vacationing in Tokyo.
It's the strongest earthquake the world has seen since the 9.1 magnitude Indian Ocean quake in 2004, which triggered a massive tsunami. The combination of the quake and tsunami left 230,000 people dead.
The Japan quake hit at 2:46 p.m. local time in Japan, and lasted an astonishing five minutes. The devastating earthquake in Northridge, Calif., in 1994 lasted just six seconds.
"The bigger the earthquake, the larger the size of the fault that has to rupture to make it happen. You're seeing waves generated along a huge fault," California Institute of Technology seismologist Kate Hutton said.
At least 125 aftershocks -- some as large as 7.4 -- have struck Japan, reversing the path of rivers, washing away boats and cars, and leaving buildings shaking.
ABC News' Michael James and the Associated Press contributed to this report.