Around 10,000 people are unaccounted for in the Japanese port of Minamisanriku, reports say, as the estimated death toll from the country's quake and tsunami nears the 1,300 mark.
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 and were 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."
<a href="http://www.coveritlive.com/mobile.php?option=com_mobile&task=viewaltcast&altcast_code=b9e2984d9d" >Tsunami Hits Japan</a>The United Nations has also sent a group to help coordinate relief work.
Attempts are being made to reduce the pressure in two reactors after Japan's prime minister Naoto Kan revealed some radiation was released.
The Japanese government has also issued another tsunami warning, telling people to move to higher ground if they can.
The country's meteorological agency warned that the waves could be more than three metres high in some places.
Japan Tsunami: Brit Talks About Experience
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: "From the first weeks of kindergarten, the children are taught how to behave in a quake so everything was very orderly."
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.
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.
The official death toll is 413, with 784 people missing and 1,128 injured. Media reports in Japan put the death toll above 1,300.
Coastal and low-lying areas in the northeastern region were inundated by the tsunami, with roads and railways engulfed and vehicles and trains swept away.