The toll in Japan's Miyagi prefecture alone is expected to top 15,000, media reports said Sunday.
The prediction came from the prefecture's police chief, the Kyodo News agency reported.
The total number of confirmed dead in Japan's magnitude-9 earthquake and tsunami disasters stood at 8,133 at 12 p.m. Sunday (0300 GMT), the National Police Agency said.
Some 12,272 people were unaccounted for, according to police.
The mayor of Otsuchi in Iwate prefecture was confirmed as dead, officials in the prefecture said Sunday. Koki Kato was washed away by the tsunami from outside a government building while holding an emergency meeting.
Some 120,000 personnel from the Self-Defence Forces, police and firefighters were involved in rescue and relief efforts.
Japanese authorities said that overseas aid that has arrived included 230,000 water bottles from South Korea, 500 generators from Taiwan and 25,000 blankets from Canada.
More than 360,000 survivors of the March 11 disasters were being housed in shelters.