Tokyo (CNN) -- The morning after Japan was struck by the most powerful earthquake to hit the island nation in recorded history and the tsunami it unleashed -- and even as the earth continued to twitch with aftershocks -- the disaster's massive impact was only beginning to be revealed.

The 8.9-magnitude temblor, which was centered near the east coast of Japan, killed hundreds of people, caused the formation of 30-foot walls of water that swept across rice fields, engulfed entire towns, dragged houses onto highways, and tossed cars and boats like toys. Some waves reached six miles (10 kilometers) inland in Miyagi Prefecture on Japan's east coast.

Buildings collapsed by the score, and numerous fires were ignited.

Hundreds more people were missing, Japanese media reported, citing local and national police. Tens of thousands of people were displaced, according to Japan's Kyodo News Agency.

Prime Minister Naoto Kan said the "enormously powerful" earthquake had caused "tremendous damage over a wide area."

The quake, which struck at 2:46 p.m. (12:46 a.m. ET), prompted the U.S. National Weather Service to issue tsunami warnings for at least 50 countries and territories.

The epicenter of Friday's main quake was located off Miyagi Prefecture, about 230 miles (370 kilometers) northeast of Tokyo, the U.S. Geological Survey said.

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Also in Miyagi, officials reported that a train had derailed and authorities had lost contact with four trains in coastal areas, Kyodo reported, citing the East Japan Railway Company.

Japanese broadcasters showed video of collapsed buildings and reported widespread power outages and transportation disruptions. In Tokyo, rail service was suspended overnight, elevated highways were shut early Saturday and surface streets remained jammed as commuters -- thousands of whom had spent the night in shelters -- tried to get to their homes in outlying areas.

Video aired by Japanese broadcaster NHK showed extensive fires in Miyagi and in the port city of Hakodate, in the southern part of Hokkaido island in northern Japan. An oil refinery was burning in Chiba Prefecture near Tokyo, according to NHK. And Kyodo News said fires could be seen in extensive areas of Kesennuma in Miyagi.

Aerial views of Kesennuma showed plumes of white smoke emanating from the center of the city and large, black areas the flames had already traversed.

In the city of Minamisoma in Fukushima Prefecture, all that was left of many structures were their foundations. Only concrete and steel buildings appeared to have withstood the wash. No people were visible in the streets of the town, whose population on Friday had been 70,000.

And a dam in Fukushima Prefecture failed, washing away homes, Kyodo reported. There was no immediate word of casualties, but the Defense Ministry said 1,800 homes were destroyed.

The National Weather Service sent a warning to 50 countries and territories it said could be affected by the tsunami.

Scores of aftershocks jarred the country Saturday, punctuated by a pair of strong earthquakes in the early morning, including one with a magnitude of 7.1 and another with a magnitude of 6.6.

Radioactive material may have leaked from an atomic power plant in northeast Japan, a major electric company said Saturday, according to a news agency report.

Citing the Tokyo Electric Power Co., Japan's Kyodo News Agency said that radioactive substances may have seeped out of the Fukushima Daiichi nuclear reactors, about 160 miles (260 kilometers) north of Tokyo.

And cooling problems appeared to have spread to another of the Tokyo Electric Power Company's nuclear plants.

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Kyodo reported the power company alerted authorities that the cooling system at three units of the Fukushima Daini plant -- which is distinct from the Fukushima Daiichi reactors -- also failed. That prompted Japanese authorities to add that plant to its emergency list, along with the Fukushima Daiichi plant, Kyodo said.

The agency also reported Saturday that the same agency ordered the power company to release a valve in the Fukushima Daiichi plant's "No. 1" building, to relieve growing pressure.

Citing Japan's nuclear safety agency, Kyodo said radiation levels were 1,000 times above normal in the the control room of the facility's "No. 1."

Prime Minister Kan told reporters he would board a helicopter to inspect the plant and the rest of the affected region from a helicopter.

The government had ordered the evacuation of residents nearest the plant as efforts to keep it cool after it was shut were initially hampered.

The confirmed death toll stood at 202 in nine prefectures, not counting the 200 to 300 bodies -- apparently drowned -- found in Sendai, Miyagi Prefecture, Kyodo said, citing police. It reported that 673 people were unaccounted for.

But NHK, also citing police, said at least 427 people were confirmed dead and more than 740 were missing across several prefectures.

Kyodo predicted the death toll would surpass 1,000.

The news agency, citing Japan's defense forces, also said 60,000 to 70,000 people were being evacuated to shelters in the Sendai area of Miyagi Prefecture.

The prime minister said an emergency task force had been activated, and he appealed for calm. The government dispatched 8,000 troops to assist in the recovery effort and asked for U.S. military assistance, according to Kyodo.

A spokesman for the U.S. military bases in Japan said all service members were accounted for and there were no reports of damage to installations or ships.

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U.S. President Barack Obama offered his condolences and said the United States was standing by to help "in this time of great trial."

The U.S. Navy initiated reconnaissance flights to map the disaster zone and was moving the aircraft carrier USS Ronald Reagan into position to assist the Japanese government with relief efforts, defense officials said.

Two search-and-rescue teams, totaling more than 140 people, were en route, the U.S. Agency for International Development said.

Images from Japanese media and CNN iReporters showed smoke pouring from buildings and water rushing across fields, carrying away entire structures.

"I wasn't scared when it started ... but it just kept going and going," said Michelle Roberts, who lives in central Tokyo. "I won't lie, it was quite scary. But we are all OK. We live on the third floor, so most everything shook and shifted."

The quake toppled cars off bridges and into waters underneath. Waves of debris flowed like lava across farmland, pushing boats, houses and trailers. About 4 million homes had no power in Tokyo and surrounding areas.

The quake also disrupted rail service and affected air travel. Hundreds of flights were canceled, Kyodo said. Some 13,000 people were stranded at the Narita airport, and 10,000 were stuck at the Haneda airport, the news agency said. Flights into and out of both airports had resumed Saturday.

At Tokyo Station, one of Japan's busiest subway terminals, shaken commuters grabbed one another to stay steady as the ground shook. Dazed residents poured into the streets, and offices and schools were closed. Children cried.

Residents said that although earthquakes are common in Japan, Friday's stunned most people.

"This was larger than anyone expected and went on longer than anyone expected," said Matt Alt, who lives in Tokyo.

"My wife was the calm one. ... She told us to get down and put your back on something, and leave the windows and doors open in case a building shifts so you don't get trapped."

Richard Lloyd Parry said he looked through a window and saw buildings shaking from side to side.

"Central Tokyo is fine from what we see, people are calm ... and not going inside buildings," he said.

Such a large earthquake at such a shallow depth -- 15.2 miles (24.5 kilometers) -- creates a lot of energy, said Shenza Chen of the U.S. Geological Survey.

The impact was felt far and wide. In McKinleyville, California, a wave swept three men into the Pacific Ocean as they were reportedly trying to take photos of the incoming tsunami waves, according to the U.S. Coast Guard.

Two of the men returned to shore, but one died, officials said.

Japanese government officials said large tsunami waves were still a risk to coastal Japan, and they urged residents in coastal areas to move to higher ground.

The tsunami brought waves of nearly 7 feet to a harbor in Maui, authorities said, but other areas reported lower levels.

On the U.S. mainland, wave heights from Alaska to California ranged from under a foot to over 8 feet. The highest measurement, 8.1 feet, was at Crescent City, California.

Tsunamis are a series of long ocean waves that can last five to 15 minutes and cause extensive flooding in coastal areas. A succession of waves can hit -- often the highest not being the first, CNN meteorologist Ivan Cabrera said.

Humanitarian agencies were working with rescue crews to reach people affected by the earthquake and tsunami.

"When such an earthquake impacts a developed country like Japan, our concern also turns to countries like the Philippines and Indonesia, which might not have the same resources," said Rachel Wolff, a spokeswoman for World Vision.

Wolff said her agency is helping people in Japan and teaming up to help others in countries along the path of the tsunami.

The quake was the latest in a series around Japan this week.

On Wednesday, a 7.2-magnitude earthquake struck off the coast of Honshu, the country's meteorological agency said. Early Thursday, an earthquake with a preliminary magnitude of 6.3 struck off the same coast.

Friday's quake is the strongest earthquake in recorded history to hit Japan, according to U.S. Geologic Survey records. The previous record was an 8.6-magnitude earthquake that struck near the Chubu Region near southwestern Honshu on October 28, 1707, that may have killed 5,000 people, CNN meteorologist Sean Morris said.

That quake generated a 33-foot (10-meter) tsunami wave, and some scientists believe the quake may have triggered the eruption of Mount Fuji 49 days later, Morris said.

The world's largest recorded quake took place in Chile on May 22, 1960, with a magnitude of 9.5, the USGS said.