Japanese engineers continue work to avoid meltdown at two nuclear power stations.
An explosion rocked a nuclear power station in Fukushima, Japan, on Saturday.
The explosion did not damage the core reactor of the plant and radiation levels were falling.
The massive 8.9-magnitude quake damaged two power plants' cooling systems and workers are using workarounds to keep the nuclear cores cool.
Two US nuclear experts are part of a broader US aid team sent to the disaster zone.
The US Nuclear Regulatory Commission said Saturday it has sent two experts to Japan, where authorities were seeking to calm fears of a reactor meltdown in the aftermath of a massive earthquake.
"We have some of the most expert people in this field in the world working for the NRC and we stand ready to assist in any way possible," commission chairman Gregory Jaczko said in a statement announcing the deployment.
The NRC -- an independent agency mandated by Congress to regulate US commercial nuclear power plants and other nuclear materials -- said the pair were experts in boiling water nuclear reactors and are part of a broader US aid team sent to the disaster zone.
The commission has activated "its Maryland-based headquarters Operations Center since the beginning of the emergency in Japan, and is operating on a 24-hour basis," the statement added.
The explosion at one of Japan's nuclear plants was not in the reactor itself, but rather was due to a failure in the pumping system as a crew was working to cool the reactor and prevent a meltdown, according to news reports.
The next step for workers at the Fukushima Daiichi atomic plant will be to, over the next two days, fill the container surrounding the reactor with seawater to return the reactor's temperature to safe levels, CNN reported Chief Cabinet Secretary Yukio Edano as saying on Saturday.
The explosion and feared meltdown exposed the scale of the disaster facing Japan after a massive quake and tsunami on Friday left 1,000 feared dead.
Reactor cooling systems failed at two plants after the record 8.9-magnitude earthquake hit. Radiation levels, which had been rising sharply, have fallen since the explosion rocked the plant, according to Kyodo News Agency. Edano said there was no immediate danger, but an area about 12.5 miles around the reactor was being evacuated as a precaution, CNN reported.
Thousands were also evacuated from near a second plant, Fukushima No. 2, which also suffered damage to its cooling system.
The earthquake unleashed a terrifying 10-meter (33-foot) wave that tore through coastal towns and cities, destroying all in its path and leaving thousands of people missing.
Smoke billowed from the Fukushima No. 1 atomic plant, about 250 kilometres (160 miles) northeast of Tokyo, after the explosion at the aging facility destroyed the walls and roof, reports said. The blast reportedly left several workers with non-life-threatening injuries.
The atomic emergency came as the country struggled to assess the full extent of the devastation wreaked by the massive tsunami, which was unleashed by the strongest quake ever recorded in Japan.
The towering wall of water pulverised the northeastern city of Sendai, where police reportedly said 200-300 bodies had been found on the coast.
More than 215,000 people were in emergency shelters, police said.
The full scale of those left homeless was believed to be much higher, with police saying they had not received a tally from Miyagi prefecture, the hard-hit province that is home to Sendai.
"What used to be residential areas were mostly swept away in many coastal areas and fires are still blazing there," Prime Minister Naoto Kan said after surveying the damage by helicopter.
The unstoppable black tide picked up shipping containers, wrecked cars and the debris of shattered homes and 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 TV reporters before breaking down in tears. "I have no words to say."
"The damage is so enormous that it will take us much time to gather data," an official at the police agency told AFP.
Authorities said more than 3,000 homes were destroyed or swept away and tens of thousands of people spent the night in emergency shelters.
The tsunami obliterated Rikuzentakata, a coastal city of some 23,000 people, the Fire and Disaster Management Agency said.
In the shattered town of Minamisoma, Sayori Suzuki, a 34-year-old housewife, recalled the utter horror of the moment the quake hit.
"It was a tremor like I've never experienced before," she told AFP. "Things just flew from the shelves."
"My house is okay, but a relative's house was washed away."
Some 50,000 military and other rescue personnel were pushed into action to spearhead the Herculean rescue and recovery effort with hundreds of ships, aircraft and vehicles headed to the Pacific coast area.
Army helicopters airlifted people off the roof of an elementary school in Watari, Miyagi prefecture.
The towering wave set off alerts across the Pacific, sparking evacuations in Hawaii and on the US West Coast.
Chile said it was evacuating coastal areas and Ecuador's state oil company announced it had suspended crude oil exports due to risks posed by the tsunami.
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.
Major manufacturers including Toyota, Nissan and Sony were forced to suspend production at some sites, 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.
Leading international offers of help, 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 40,000 military personnel in Japan, ordered a flotilla including two aircraft carriers and support ships to the region to provide aid.
The quake, which hit at 2:46 pm (0546 GMT) and lasted about two minutes, rattled buildings in greater Tokyo, the world's largest urban area and home to some 30 million people.
Millions were left stranded in the evening after the earthquake shut down the city's vast subway system.
But with small quakes felt every day somewhere in Japan, the country is one of the best prepared to deal with the aftermath of such a calamity.
"If there is any place in the world ready for a disaster of the scale and scope of this historic calamity, it is Japan," said Stacey White, senior research consultant at the Centre for Strategic and International Studies.
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.
But mostly the picture was one of utter devastation.
The tsunami submerged the runway at Sendai airport, while a process known as liquefaction, caused by the intense shaking of the tremor, turned parts of the ground to liquid.
Hours after the quake struck, TV images showed huge orange balls of flames rolling up into the night sky as fires raged around a petrochemical complex in Sendai. A massive blaze also engulfed an oil refinery near Tokyo.
Nearly 24 hours after the first, massive quake struck just under 400 kilometres northeast of Tokyo, aftershocks were still rattling the region, including a strong 6.8 magnitude tremor on Saturday.
The US Geological Survey said more than 100 aftershocks had hit the area.
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 will strike within the next 30 years in the Kanto plains, home to Tokyo's vast urban sprawl.