Three days after a devastating earthquake unleashed a tsunami in which at least 10,000 people are feared dead, Japan on Sunday faced a deepening nuclear crisis and the prospect of another very powerful quake.

The prime minister, Naoto Kan, called the disaster "Japan's most severe crisis since the war ended 65 years ago". He called on the country to unite and said its future would be decided by the response to this crisis.

The threat of further seismic shifts and tsunami is far from over. As rescue teams from more than 70 countries and tens of thousands of Japanese troops descended on the disaster zone, meteorological agency officials warned there was a 70% chance of a magnitude 7.0 earthquake striking the region in the next three days. "There will be many aftershocks in multiple locations. We have to brace ourselves for aftershocks of magnitude 5 or even magnitude 6," an agency official said.

The meteorological agency upgraded last Friday's earthquake, the fifth biggest in the world for a century, from magnitude 8.8 to 9.0. What began as a violent seismic shift lasting minutes has left a permanent geological legacy: the US geological survey said the force of the quake had shifted the island a distance of 8ft (2.5 metres).

New images from Japanese TV revealed, one submerged community after the other, the full extent of the devastation, and the determination of a nation united by a natural catastrophe that almost defies comprehension.

Rescue teams and survivors are in constant fear of yet more powerful quakes and tsunami. Millions of others were bracing for a potentially catastrophic nuclear accident after the government warned that a second nuclear reactor at the Fukushima Daiichi (No 1) nuclear power plant could explode.

More than 300,000 people are living in emergency shelters along the coastline and millions of survivors face a third night without water, electricity and proper food. Japanese media reported that 5.5 million people were without power, while 20,800 buildings had been destroyed or damaged.

Land once occupied by towns and villages now resembles paddy fields, the surface of the water breached by the roofs of wrecked homes, trees and other debris created by a wall of water so high that taking refuge on a building's third floor proved futile for some.

The arrival of Japanese TV crews in the worst-affected areas has given the disaster what it lacked in the immediate aftermath: a human face.

Residents broke off from searches for lost friends and relatives, and some spoke from hospital beds to recount the terror of Friday afternoon. "Everything has turned upside down," an elderly man in Miyagi prefecture said. "After the tsunami, everything had gone. But we're going to have to get on with life."

Fighting back tears, one woman said: "I tried to run away but I was just swallowed up by the tsunami. I kept shouting 'Save me, save me'. I can't begin to describe what I saw."

Another evacuee from Miyagi said: "My children are cold and complaining of headaches. We don't know how long we're going to have to stay here. It's very upsetting."

The official death toll has reached more than 700, with more than 10,000 others still missing. Given the size of the disaster, in breadth and intensity, the eventual number of dead could be much higher.

The apocalyptic images of entire neighbourhoods filled with the muddy remnants of the tsunami suggest that there is little hope that many, if any at all, will be found alive.

In Rikuzentaka, a city close to the coast, some among the 1,000 people taking refuge at a school located on high ground wept as they stared at a list of the survivors. "There have been tsunami before but they were just small. No one ever thought that it could be like this," said Michiko Yamada, a 75-year-old in Rikuzentakata, a near-flattened village in far-northern Iwate prefecture.

Images taken from the hills overlooking Kesennuma, in Miyagi prefecture, showed a town of more than 70,000 reduced to ashes by fires that raged on Friday night.

In scenes repeated along the coast, debris hangs from tree branches well inland, indicating how high the waters surged before they began to recede. One image, broadcast across the world, shows a solitary white car perched on the roof of a house.

All other news has been pushed aside while the country's broadcast media feeds an insatiable public need for information about the death toll and the nuclear crisis in Fukushima.

With alarming frequency, broadcasts are punctuated with a now-familiar jingle announcing more powerful aftershocks that snake their way down from the northeast to Chiba and Tokyo.

While Nagatacho, Japan's political nerve centre, has united around the rescue and relief effort, criticism of the authorities' response is seeping through. A headline in the Asahi Shimbun blasted the government's "incoherent" crisis management strategy, accusing it of taking too long to release information about the problems at Fukushima nuclear plant and evacuate tens of thousands of people living nearby. "Every time they urged us to 'stay calm' without providing concrete data, they simply made people more anxious," the paper quoted an unidentified politician as saying.

The cost of the rescue, relief and recovery effort will be huge. Manufacturers have closed plants while the energy infrastructure, from closed or crippled nuclear plants to burning oil refineries, is so badly damaged that power companies have warned of sporadic electricity cuts in areas hundreds of miles from the epicentre.

Russia, a rival for new energy sources in the Asia-Pacific, on Sunday offered to provide emergency supplies of natural gas.

The government, meanwhile, is poised to dip into a 200bn yen (£1.5bn) contingency fund to pay for the relief effort.

The prime minister, Naoto Kan, until Friday the subject of a funding scandal that could have cost him his job, called on the country to come together in its time of need. "We must do all we can to save as many people as possible," he said.

That effort is now an international concern, with the arrival over the weekend of rescue and medical teams from several countries, as well as a US aircraft carrier.

A team of 63-strong British rescue workers, some fresh from searching through the rubble in Christchurch, was due to land in Tokyo, along with two search dogs and 11 tonnes of equipment, including heavy lifting and cutting gear.