Defence Minister Toshimi Kitazawa said the 100,000 troops -- about 40 percent of the armed forces -- would be fully deployed in a day or two.
"There are so many people who are still isolated and waiting for assistance. This reality is very stark," Kyodo News quoted him as saying.
In the small port town of Minamisanriku alone some 10,000 people were unaccounted for -- more than half the population -- public broadcaster NHK reported.
Police and military reported finding groups of hundreds of bodies at locations along the shattered coastline, including more than 200 found at a new site on Sunday.
Offers of help came from all over the world including traditional rivals.
Despite last year's territorial row that soured relations, Chinese Premier Wen Jiabao and Foreign Minister Yang Jiechi sent condolences. A team of 15 Chinese rescuers was to arrive Sunday afternoon, Japan's foreign ministry said.
It said 69 nations or regions and five international aid organisations had offered help as of Sunday morning. Among them was New Zealand, which suffered its own quake tragedy last month in the city of Christchurch.
A 66-strong Japanese team which has spent more than two weeks searching the rubble in Christchurch was due back home to confront its own tragedy.
The Japanese Red Cross on Saturday sent 62 emergency teams to rescue victims, according to the international Red Cross federation, with around 400 doctors, nurses and other experts deployed.
Search and rescue teams and sniffer dogs arrived from Germany and Switzerland, and rescuers from Britain and France were on the way.
The aircraft carrier USS Ronald Reagan arrived off the coast early Sunday. Japan has asked for its help to refuel Japanese helicopters and transport its troops to affected areas, the US Pacific Fleet said.
A 144-member rescue team of the US Agency for International Development was also due at Misawa later Sunday to join inland operations. The team includes 12 dogs trained to detect victims trapped under rubble and about 150 tonnes of rescue equipment, USAID said.
Australia, South Korea and Singapore also pledged dogs and search and rescue teams. Australia has offered field hospitals and disaster victim identification teams, said Foreign Minister Kevin Rudd.
They had also offered the expertise of nuclear specialists to help address a threat from a damaged nuclear power station, Rudd added.
Two experts from the US Nuclear Regulatory Commission were headed for Japan, where tsunami and quake damage to one plant led to the evacuation of tens of thousands of people.