A hydrogen explosion reportedly ripped through another reactor at the Japanese nuclear plant where a reactor exploded Saturday, deepening a crisis government officials are calling the worst the nation has faced since World War II.
TV Asahi reported that the explosion at Unit 3 of the Fukushima Daiichi Plant, which officials had warned could happen after Unit 1 exploded on Saturday.
Officials from Japan's Nuclear and Industrial Safety Agency (NISA) said the reactor's containment was not damaged and although radiation was leaked, levels were low.
NISA officials also report that reactor no. 2 at Daiichi plant has lost its cooling ability and pressure is rising.
The news came as Japanese officials issued and then quickly canceled a tsunami warning, following aftershocks along the already earthquake-ravaged eastern coast of the nation.
Japanese authorities have been working frantically to prevent a meltdown at a series of nuclear reactors in Fukushima. The U.S. Nuclear Regulatory Commission has sent two of its officials with expertise in boiling water nuclear reactors.
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Unit 3 at the Fukushima Daiichi plant had been one focus of concern, and Chief Cabinet Secretary Yukio Edano had said the government knew an explosion there was possible.
Workers had tried releasing radioactive air and injecting sea water to reduce pressure and cool the reactor down to avoid an explosion like the one Saturday, an explosion at Unit 1 that injured four workers.
Already, at least 170,000 people have been evacuated in the 12 mile radius around the Fukushima plants. It is unclear if people are being asked to evacuate around the Miyagi power plant. Doctor Michio Kaku, a physicist, said that Japan should consider extending the evacuation orders.
"Winds don't stop blowing at 12 miles...computer models show that the radiation doesn't disperse in a sphere or a circle. It disperses in a plume, a pencil-like plume that then waves with the wind like a lighthouse," Kaku said.
Japanese authorities have declared a state of emergency at another nuclear power plant following Friday's massive earthquake that has left the country in a crisis Japan hasn't seen since World War II.
A state of emergency was declared at the Onagawa nuclear power plant, located in the hard-hit Miyagi prefecture, the International Atomic Energy Agency reported.
Following Friday's 8.9 magnitude earthquake, a fire broke out at the Onagawa plant but was later contained, the Associated Press reported. Officials from the Tohoku Electric Power Company said that higher than normal radioactivity readings prompted the heightened alert Sunday. The emergency is at level one, the lowest state of emergency.
Officials in Miyagi are still digging through smoldering debris and collapsed homes and buildings left in the aftermath of Friday's 8.9 magnitude earthquake and tsunami. One official estimates that up to 10,000 people could be dead in Miyagi alone.
At least 1,596 people have been killed, according to NHK News.
Officials anticipate another earthquake of 7.0 magnitude or greater in the coming days, possibly further damaging the already fragile nuclear reactors.
"If there's a secondary earthquake, that could tip the whole thing over. Pipes could break, leaks could take place and even as you put sea water in, the water could bleed out, creating a full scale meltdown. That's the nightmare scenario," Kaku said.
Another nuclear complex, the Tokai Dai-Ni plant, experienced a failure after Friday's quake, the Associated Press reported. It's not clear why the incident wasn't reported by the Japan Atomic Power Co. until Sunday.
Japanese Prime Minister Naoto Kan said in a television address Sunday that the country is facing the most difficult crisis since World War II, but he is confident the nation can overcome this disaster.
"We Japanese people have overcome all kinds of hardships and were able to create a prosperous society. In the face of the earthquake and tsunami we should be able to overcome these hardships. We believe we can overcome this," Kan said.
Kan said 100,000 troops -- plus 2,500 police, 1,100 emergency service teams, and more than 200 medical teams -- have been deployed for recovery efforts.
Millions of the country's residents are grappling with food shortages, power outages and the collapse of basic services.
Ichiro Fujisaki, the nation's U.S. ambassador, said about 2.5 million households -- just over 4 percent of all households in Japan -- were without electricity Sunday, and 500,000 homes were without water.
The government is going to begin further rationing electricity by implementing rolling blackouts.
At least 49 countries along with numerous aid organizations have mobilized relief efforts.
The nuclear-powered aircraft carrier USS Ronald Reagan arrived off the coast of Honshu Saturday, one of a number of U.S. vessels diverted to offer disaster aid to the shattered country.
At least four more Navy ships will be arriving in the days to come to assist with relief efforts.
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.
A Navy P-3 maritime surveillance plane did a survey mission earlier Sunday and discovered a huge debris field 8 miles east of the Japanese coast line. Houses, barges, oil slicks, capsized boats, and cars fill the mile wide debris field.
The U.S. Embassy said that 100,000 Americans are known to be in Japan, and 1,300 of them live in the areas most affected by the earthquake and tsunami. There are still no known American casualties.
ABC News' Michael James, Dan Childs and Dean Schabner and the Associated Press contributed to this report.