People watch the aftermath of tsunami tidal waves covering a port at Kesennuma in Miyagi Prefecture, northern Japan, after strong earthquakes hit the area.

Sendai, Japan - An explosion at a Japanese nuclear plant triggered fears of a meltdown on Saturday after a massive earthquake and tsunami left more than 1,000 dead and at least 10,000 unaccounted for.

As workers doused the stricken reactor with sea water to try to avert catastrophe, Japan's Prime Minister Naoto Kan said the chaos unleashed by Friday's 8.9 magnitude quake was an “unprecedented national disaster”.

The quake, one of the biggest ever recorded, unleashed a terrifying tsunami that engulfed towns and cities on Japan's northeastern coast, destroying everything in its path.

In the small port town of Minamisanriku alone, about 10,000 people are unaccounted for - more than half the population -

public broadcaster NHK reported.

Even as Japan struggled to assess the full extent of the devastation, the nation faced an atomic emergency as cooling systems damaged by the quake failed at two nuclear reactors.

Smoke billowed from the Fukushima No. 1 atomic plant about 250 kilometres (160 miles) northeast of Tokyo, after an explosion blew off the roof and walls of the structure around the reactor.

Radiation leaked from the plant, but the government moved to calm fears of a meltdown, saying that the blast did not rupture the container surrounding the reactor and that radiation levels had fallen afterwards.

Japan's nuclear safety agency rated the accident 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.

Work to bring the situation under control was ongoing, to prevent cooling liquid from evaporating and exposing the fuel rods to the air, which could trigger a major radiation leak.

“We have decided to douse the (reactor) container with sea water in order to reduce risks as quickly as possible,” Kan's top spokesman Yukio Edano told reporters.

Kyodo and Jiji reported before the explosion that the plant “may be experiencing nuclear meltdown”, while NHK quoted the safety agency as saying metal tubes that contain uranium fuel may have melted.

An evacuation order for tens of thousands of residents was expanded to 20 kilometres (12 miles) around the Fukushima plant, and thousands more were shifted from another damaged plant, Fukushima No. 2.

Media reports said three residents - bedridden patients evacuated from a hospital near the plant - had been found to be exposed to radiation after spending a long time outdoors awaiting rescue.

The number of people exposed to radiation was expected to climb to at least 90, the Asahi Shimbun and Yomiuri Shimbun newspapers said.

Takashi Fujimoto, vice president of nuclear plant operator Tokyo Electric Power Co., apologised for the accident, telling reporters: “We gave people a lot of trouble.”

Ron Chesser, director of the Center for Environmental Radiation Studies at Texas Tech University, said it was critical to cool the reactor core to avoid a meltdown that would result in “a large release of radiation”.

“Reactors are not like your car that you can turn off and walk away. They're going to continue generating a great amount of heat until the core is disassembled,” he told the US-based ScienceDaily website.

The raging tsunami picked up shipping containers, cars and the debris of shattered homes. It crashed through the streets of Sendai and across open fields, forming a mud slick that covered vast tracts of land.

“There are so many people who lost their lives,” an elderly man told television reporters before breaking down in tears. “I have no words to say.”

Police reportedly said between 200 and 300 bodies had been found in the city of Sendai. Up to 400 bodies were recovered in Rikuzentakata, a coastal town of some 23,000 people, NHK quoted the military as saying.

The premier's spokesman said at least 1,000 people were believed to have lost their lives. Police said more than 215,000 people were huddled in emergency shelters.

“What used to be residential areas were mostly swept away in many coastal areas and fires are still blazing there,” Kan said after surveying the damage by helicopter.

In the shattered town of Minamisoma, 34-year-old housewife Sayori Suzuki recalled the utter horror of the moment the quake hit, shaking her home violently and washing away the house of a relative.

“It was a tremor like I've never experienced before,” she told an AFP reporter.

“Another relative said he was fleeing in a car but watched in the rear-view mirror as the waves were catching up with him from behind. He escaped very narrowly.”

“My brother works at the Fukushima No.2 nuclear power plant,” Suzuki added. “He worked all through the night. I'm so worried about him because of the risk of radiation exposure.”

About 50,000 military and other rescue personnel were spearheading a Herculean rescue and recovery effort with hundreds of ships, aircraft and vehicles headed to the Pacific coast area.

The towering wave set off alerts across the Pacific, sparking evacuations in Hawaii and on the US West Coast. Tsunami waves destroyed some coastal buildings in Peru but otherwise had little effect on Latin America.

The Bank of Japan said it would do its “utmost” to ensure the stability of financial markets after the quake brought huge disruption to key industries, raising short-term concerns for the nation's struggling economy.

In quake-hit areas, 5.6 million households had no power Saturday and more than one million households were without water. Telecommunications networks were also hit.

In a rare piece of good news, a ship that was earlier reported missing was found swept out to sea and all 81 people aboard were airlifted to safety.

Leading international offers of help, which saw foreign rescue teams begin arriving in Japan on Saturday, President Barack Obama mobilised the US military to provide emergency aid after what he called a “simply heartbreaking” disaster.

The United States, which has nearly 50,000 military personnel in Japan, ordered a flotilla including two aircraft carriers and support ships to the region to provide aid.

The quake hit at 2:46 pm (0546 GMT) and lasted about two minutes, making buildings sway in greater Tokyo, the world's largest urban area and home to some 30 million people.

More than a day after the first massive quake struck just under 400 kilometres north-east of Tokyo, aftershocks were still rattling the region, including a strong 6.8 magnitude tremor on Saturday.

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.

The government has long warned of the likelihood that a devastating magnitude eight quake would strike within the next 30

years in the Kanto plains, home to Tokyo's vast urban sprawl. - Sapa-AFP