(CNN) -- Rescuers fanned out across northern Japan on Monday, in efforts coordinated and improvised, attempting to reach untold numbers of people still stranded after a massive earthquake and tsunami shattered the region.
A sense of urgency prevailed among responders as a third frigid night fell upon the survivors of Saturday's 8.9-magnitude quake, the most powerful measured seismic event in Japan's history.
Weather forecasts called for continued temperatures barely above freezing, as well as rain and freezing precipitation that could trigger mudslides. Continued subnormal cold also will probably strain power generation in a country already employing rolling blackouts as a conservation measure.
In areas cut off from the outside world by the disaster, more than 450,000 people whose homes are lost or inaccessible were staying in shelters, according to NHK, Japan's national broadcasting company.
Many survivors set out on their own in search of friends and loved ones.
NHK showed video of a business owner in Iwate Prefecture, who watched the tsunami consume his town and has spent the days since trying to contact his employees, choking back tears as he came upon a worker who was unhurt. He said he had located only 22 of his 50 employees by Monday morning but had found three more that day.
A man riding a bicycle told the network he was looking for his wife. Her name was written on a sign attached to his bike, and he was carrying photos of her from place to place, showing them to everyone he met.
Meanwhile, help from abroad continued to arrive. The Japanese Foreign Ministry said rescuers from 11 countries have arrived, and others are en route from France and Russia.
President Barack Obama, in brief remarks during a school event in Virginia, promised that the United States "will continue to offer any assistance we can. ... And we will stand with the people of Japan in the difficult days ahead."
Danish Prime Minister Lars Lokke Rasmussen, meeting with Obama on Monday, said the relief efforts for Japan would also now be a key part of his discussions with the president. Rasmussen said he had sent a letter to Japanese Prime Minister Naoto Kan on Friday, offering assistance.
A U.S. search-and-rescue convoy made an anxious and arduous seven-hour overland journey from northeastern corner of Honshu, Japan's main island, to the outskirts of Ofunato in Iwate Prefecture, only to be told it was too dangerous to unload and set up their base camp in the darkness.
"They wanted to get on the ground earlier than this," said CNN correspondent Brian Todd, one of four journalists embedded with the search-and-rescue team. "By the time they set up that base camp and fan out into Ofunato, it will have been 90 hours since the earthquake."
The 150-member team, accompanied by 12 search dogs and five flatbed trucks full of equipment, was dispatched by the U.S. Agency for International Development (USAID). The team is based in Fairfax, Virginia, and is joined by members the Los Angeles County Search and Rescue Team.
A Chinese team also has begun operations in Iwate Prefecture, and a South Korean rescue team is at work in Miyagi Prefecture.
Other nations with rescue teams on the ground are Singapore, Germany, Switzerland, New Zealand, Britain, Australia, Mexico and Taiwan.
The foreign ministry said 91 countries and territories have offered assistance. In addition, six international organizations, including the World Food Programme and the International Committee of the Red Cross, also have offered their support.
The Fire and Disaster Management Agency said the number of buildings that were destroyed or heavily damaged stood at 63,255 as of Monday afternoon, the Kyodo news agency reported.
Relief teams still were trying to reach isolated areas in northern Tohoku region, including 800 people sheltered in a gym and 600 in a shopping center, NHK reported.
In flooded Ishinomaki, a city of about 160,000 northeast of the hard-hit city of Sendai, Japanese civil defense teams set out in rowboats and inflatable rafts during an all-day operation to retrieve hundreds of people stranded in buildings and get them to shelter.
The difficulties facing search and rescue teams continued to compound in about every way imaginable: shattered infrastructure, relentless and powerful aftershocks, and a brewing crisis at a nuclear power plant.
On Monday, the USS Ronald Reagan moved to a different location off Japan's northeastern coast after readings indicated heightened radiation from the damaged Fukushima Daiichi nuclear plant. Also, tests detected low levels of radioactivity on 17 U.S. Navy helicopter crew members when they returned to the aircraft carrier. (No further contamination was detected after the crew members washed with soap and water, the Navy said).
In the coming days, the worsening weather will be an even more serious factor.
CNN International Meteorologist Mari Ramos said cloud cover is beginning to move in over Japan, with rain expected beginning Tuesday and even lower temperatures and windy conditions -- with possible freezing rain and snow -- on Thursday.
Heavy precipitation and winter mix would present the threat of mudslides and avalanches. Though landslides aren't a concern in low-lying areas affected by the tsunamis, they will be a deep source of concern in parts of the nation's interior that were "badly shaken" by the quake and aftershocks, Ramos said.
In other more severely affected areas, rescue efforts gave way to forlorn recovery.
About 1,000 bodies were found Monday on several shores on the Oshika Peninsula in Miyagi Prefecture, and police and firefighters worked to recover another 200 to 300 bodies in Sendai, the Kyodo news agency reported.
Ominous reports had emerged over the weekend from Minami Sanriku, where more than half of the 17,000 residents had not been accounted for. On Monday, the visuals seemed to affirm the horrifying math as recovery teams sifted through an utterly ruined and silent city, looking for the dead and signs of life.
Minami Sanriku is nestled at the end of Shizugawa Bay, a 2-mile-wide, 5-mile-long body of water that opens into the Pacific Ocean.
In Ishinomaki, CNN correspondent Gary Tuchman and a film crew set out with civil defense soldiers rescue team in the flooded city.
En route, they saw people flagging them from windows of houses. One woman called out that she needed water.
During that rescue sortie, the team arrived at a large building where floodwaters had turned an entrance ramp into a convenient boat ramp, where a handful of survivors were waiting. Soldiers fitted them with life jackets and helped them aboard.
On the return trip, the rowboat passed other rescue teams in empty boats paddling to other locations to rescue the stranded.
The team later arrived at a Red Cross shelter, where weakened survivors were gently placed on stretchers and carried away.
NHK showed a civil defense soldier carrying a woman into a room in a shelter. After he gingerly set her down, the woman rose to her feet with some difficulty and bowed to the soldier, told him she was all right, bowed again and then collected herself to briefly tell her story, paraphrased by an NHK interpreter:
"She had been waiting for help all night, outside. She had been washed away by the wave. ... The moment she opened the door of the house, the water flooded in. ... She grabbed hold of a tree and hung on, hung on for dear life with the water all around her. A ... floor mat floated by, and she grabbed it and held on to that."
As the woman spoke in Japanese, the interpreter's voice trembled in English: "Her daughter was washed away. She was washed away, and she has not found her."
CNN's Brian Todd, Justine Redman and Gary Tuchman contributed to this report.