Around 10,000 people are reportedly unaccounted for in the Japanese port of Minamisanriku, as the estimated death toll from the country’s quake and tsunami nears the 1,700 mark.
The confirmed death toll from the combined disasters stands at 686, while at least 1,000 people have been injured.
Japanese military have also said 300 to 400 bodies have been found in Iwate’s Rikuzentakata city.
The country has asked for Britain’s help with a rescue operation of unprecedented proportions.
The foreign office confirmed they had offered assistance in the form of victim identification expertise, humanitarian aid, and expert disaster search and rescue teams.
British rescue workers are on their way to the country.
Foreign secretary William Hague said he had spoken to the Japanese foreign minister to offer his condolences.
“We are appalled by the scenes of devastation, by the heavy loss of life, by the destruction we have all witnesses on our television screens.
“I think all over the world, people’s hearts go out to the people of Japan.”
The United Nations has also sent a group to help coordinate relief work, while a 66-strong Japanese team has been making hasty preparations to return from New Zealand.
The team had been searching through rubble in the wake of the earthquake in Christchurch.
The US is also sending 150 rescue workers to the disaster zone, among them a team from Los Angeles that only returned from New Zealand two days ago.
President Barack Obama has ordered emergency aid and an aircraft carrier to be sent to the country, while China’s Red Cross has also pledged 1m Yuan (£94,000) in aid.
Charities in Japan are also helping with the relief work. Mark Pearson, from Shelterbox, said they had teams flying in from Australia, the UK and the US and were distributing their survival kits.
“We have had a lot of experience in Haiti and Pakistan, but this is proving to be a unique set of circumstances,” he said.
Mr Pearson estimated that 200,000 people were displaced in the northern areas of the country.
Tens of thousands of residents have been evacuated from an area near two nuclear plants in Fukushima province, some 150 miles (240km) north of Tokyo.
British holidaymaker Scott Myers told Sky News he was on a train when the quake hit.
“After the quake, the train stopped and all the power went off.
“They got us off the train with ladders and walked us up the tracks because none of the trains were moving.
“The knock on effect from that later in the night was that lots of commuters all around Tokyo could not seem to get anywhere.
“There were people bedding down in the subway tunnels, in shopping centres and just clambering into hotel receptions and lying on the floor, just trying to get some shelter really.”
British teacher Nick George told Sky that in his 14 years in the country he had never experienced a quake so strong.
He was in the school building when the quake struck and said it “creaked, but nothing broke”.
The children, he said, were calm and sat under tables, while the teachers donned hard hats: “From the first weeks of kindergarten, the children are taught how to behave in a quake so everything was very orderly,” he said.
Meanwhile, more than 215,000 people are being put up in emergency shelters in the east and north of the country.
The number of homeless is believed to be much higher, as police said they had not received a tally from Miyagi province, a northern area where hundreds of deaths are reported.
Transport within the country is difficult, and all highways from Tokyo to affected areas are closed except to emergency vehicles. Some areas have also been rendered unreachable by water.
Local television has shown people lining up for water and food such as rice balls in various locations, while in Fukushima, city officials have been handing out bottled drinks, snacks and blankets.
Residents in Sendai have reportedly been hoarding water and huddling in makeshift shelters in near-freezing temperatures.
In Mito city, residents were seen queuing outside a damaged supermarket for supplies.
“All the shops are closed, this is one of the few still open,” said Kuno Iwatsuki.
“I came to buy and stock up on diapers, drinking water and food.”
Friday’s massive quake had a magnitude of 8.9. It caused a 33ft wave that hit the port of Sendai city, sending ships crashing into the shore and carrying cars and buildings through streets.
Amid the destruction in Japan came a more joyful story, as naval and coastguard helicopters found a ship that had been swept out to sea and airlifted all 81 people aboard to safety.
The quake was the country’s biggest on record – around 8,000 times more powerful than the one that devastated Christchurch in New Zealand last month.
A number of aftershocks have been felt throughout the Pacific basin, with a 6.8-magnitude quake off the coast of Japan itself.
A further aftershock hit near Fukushima Number 1 plant, with a magnitude of 6.4.
The small South Pacific islands of Tonga also experienced a 6.1-magnitude tremor.
Britain’s Foreign and Commonwealth Office has advised against all non-essential travel to Tokyo and the north east of Japan, whilst it assesses the damage caused.
Media in the country reported widespread fires in the northeastern city of Kesennuma, which has a population of 74,000 and where one-third of the city was submerged.
Leaders of Japan’s ruling and opposition parties have agreed on the need to compile an extra budget and bankers pledged to do their utmost to ensure financial market stability.
Insurance analysts believe the total insured loss could be around £10bn.