SENDAI, Japan -- An explosion at a nuclear power station tore down the walls of one building Saturday as smoke poured out and Japanese officials said they feared the reactor could melt down following the failure of its cooling system in a powerful earthquake and tsunami.
It was not clear if the damaged building housed the reactor. Tokyo Power Electric Co., the utility that runs the Fukushima Dai-ichi plant, said four workers were injured but details were not immediately available.
Footage on Japanese TV showed that the walls of one building had crumbled, leaving only a skeletal metal frame standing. Puffs of smoke were spewing out of the plant.
"We are now trying to analyze what is behind the explosion," said government spokesman Yukio Edano, stressing that people should quickly evacuate a six-mile (10-kilometer) radius. "We ask everyone to take action to secure safety."
The trouble began at the plant's Unit 1 after Friday's massive 8.9-magnitude earthquake and the tsunami it spawned knocked out power there. The disaster has killed hundreds of people and devastated the country's northeastern coast, where rescuers began slowly arriving Saturday.
The toll of destruction was still not known more than 24 hours after the quake since washed-out roads and shut airports have hindered access to the area. An untold number of bodies were believed to be buried in the rubble and debris.
The official death toll stood at 413, while 784 people were missing and 1,128 injured. In addition, police said between 200 and 300 bodies were found along the coast in Sendai, the biggest city in the area near the quake's epicenter. Local media reports said at least 1,300 people may have been killed.
Adding to worries was the fate of nuclear power plants in the region. Japan has declared states of emergency for five nuclear reactors at two power plants after the units lost cooling ability.
The most troubled one is facing meltdown, officials have said.
Pressure has been building up in the reactor – it's now twice the normal level – and Japan's Nuclear and Industrial Safety Agency told reporters Saturday that the plant was venting "radioactive vapors." Officials said they were measuring radiation levels in the area.
The reactor in trouble has already leaked some radiation: Operators have detected eight times the normal radiation levels outside the facility and 1,000 times normal inside Unit 1's control room.
Wind in the region is weak and headed northeast, out to sea, according to the Meteorological Agency.
Ryohei Shiomi, an official with Japan's nuclear safety commission, said that even if there was a meltdown, it wouldn't affect people outside a six-mile (10-kilometer) radius – an assertion that might need revising if the situation deteriorates. Most of the 51,000 residents living within the danger area had been evacuated, he said.
Meanwhile, the first wave of military rescuers began arriving by boats and helicopters.
Prime Minister Naoto Kan said 50,000 troops would join rescue and recovery efforts following the quake that unleashed one of the greatest disasters Japan has witnessed – a 23-foot (7-meter) tsunami that washed far inland over fields, smashing towns, airports and highways in its way.
"Most of houses along the coastline were washed away, and fire broke out there," said Kan after inspecting the quake area in a helicopter. "I realized the extremely serious damage the tsunami caused."
More than 215,000 people were living in 1,350 temporary shelters in five prefectures, or states, the national police agency said. Since the quake, more than 1 million households have not had water, mostly concentrated in northeast.
The transport ministry said all highways from Tokyo leading to quake-hit areas were closed, except for emergency vehicles. Mobile communications were spotty and calls to the devastated areas were going unanswered .
Local TV stations broadcast footage of people lining up for water and food such as rice balls. In Fukushima, city officials were handing out bottled beverages, snacks and blankets. But there were large areas that were surrounded by water and were unreachable.
One hospital in Miyagi prefecture was seen surrounded by water. The staff had painted an SOS on its rooftop and were waving white flags.
Prime Minister Kan said a total of 190 military aircraft and 25 ships have been sent to the area, which continued to be jolted by tremors, even 24 hours later.
More than 125 aftershocks have occurred, many of them above magnitude 6.0, which even alone would be considered strong.
Technologically advanced Japan is well prepared for quakes and its buildings can withstand strong jolts, even a temblor like Friday's, which was the strongest the country has experienced since official records started in the late 1800s. What was beyond human control was the killer tsunami that followed.
It swept inland about six miles (10 kilometers) in some areas, swallowing boats, homes, cars, trees and even small airplanes.
"The tsunami was unbelievably fast," said Koichi Takairin, a 34-year-old truck driver who was inside his sturdy four-ton rig when the wave hit the port town of Sendai.
"Smaller cars were being swept around me," he said. All I could do was sit in my truck."
His rig ruined, he joined the steady flow of survivors who walked along the road away from the sea and back into the city on Saturday. Smoke from at least one large fire could be seen in the distance.
Smashed cars and small airplanes were jumbled up against buildings near the local airport, several miles (kilometers) from the shore. Felled trees and wooden debris lay everywhere as rescue workers coasted on boats through murky waters around flooded structures, nosing their way through a sea of debris.
Basic commodities were at a premium. Hundreds lined up outside of supermarkets, and gas stations were swamped with cars. The situation was similar in scores of other towns and cities along the 1,300-mile-long (2,100-kilometer-long) eastern coastline hit by the tsunami.
In Sendai, as in many areas of the northeast, cell phone service was down, making it difficult for people to communicate with loved ones.
"I'm waiting for my son to come here. But I cannot tell him he should come over here because mobile phones aren't working," a woman in her 70s told Japanese TV at a shelter in the town of Rikuzentakada, which appeared to be largely destroyed by the tsunami.
"My husband is missing," she said. "Tsunami water was rising to my knees, and I told him I would go first. He is not here yet."
President Barack Obama pledged U.S. assistance following what he called a potentially "catastrophic" disaster. He said one U.S. aircraft carrier was already in Japan and a second was on its way. A U.S. ship was also heading to the Marianas Islands to assist as needed, he said.
Most trains in Tokyo started running again Saturday after the city had been brought to a near standstill the day before. Tens of thousands of people had been stranded with the rail network down, jamming the streets with cars, buses and trucks trying to get out of the city.
Japan's worst previous quake was a magnitude 8.3 temblor in Kanto that killed 143,000 people in 1923, according to the USGS. A magnitude 7.2 quake in Kobe killed 6,400 people in 1995.
Japan lies on the "Ring of Fire" – an arc of earthquake and volcanic zones stretching around the Pacific where about 90 percent of the world's quakes occur, including the one that triggered the Dec. 26, 2004, Indian Ocean tsunami that killed an estimated 230,000 people in 12 countries. A magnitude-8.8 quake that shook central Chile in February 2010 also generated a tsunami and killed 524 people.
Associated Press writers Malcolm J. Foster, Mari Yamaguchi, Tomoko A. Hosaka and Shino Yuasa in Tokyo contributed to this report.