On the Dominican side, another 23 bodies were recovered overnight near the border town of Jimani, where corpses were washing downstream into a saltwater lake crowded with crocodiles.
In Haiti, bodies floated near the tops of palm trees, and thousands of survivors were isolated by mudslides. Rains returned Thursday and forecasters were predicting more for Friday.
"There's one town that people described as just like a lake," reporter Amy Bracken told CBS Radio News. "There are all these houses that are submerged, and they're assuming that there are families in those houses, and the relatives haven't claimed them, the relatives haven't put their name on the list as missing. There was one woman from that area who said there must be more than 1,000 who are dead in this lake area."
With few roads passable and only 14 helicopters, troops were trying to find other ways to reach the needy, said Lt. Col. Dave Lapan, spokesman for the U.S-led multinational force in Haiti.
Helicopters that went in Friday carried inflatable rubber boats and shovels and pick axes to allow troops and relief workers to reach far-flung villages, he said.
The U.N. World Food Program loaded some 8,000 tons of food onto a helicopter bound on Friday for the southern town of Mapou, where most houses were under water and some 1,000 people were feared dead.
The WFP was considering using barges to bring more food to coastal villages. "The need is extreme," said Guy Gavreau, a spokesman for WFP.
"The shelter that they are beginning to provide .... is sheets of plastic particularly provided by the U.N., and it will just be very minimal shelter for people trying to get out of the rain," said Bracken.
The Haitian government's official death toll climbed to 579 as about 165 people, including 45 children, were declared dead in the border town of Fond Verrettes. Few bodies were found there after floods swept away most houses.
At least 442 bodies were recovered in the Dominican Republic, a number of them Haitian migrants who had crossed over to work as sugar cane cutters or market vendors.
The hundreds of missing fed fears that the final toll could climb as high as 2,000 in the deadliest floods to hit Hispaniola island in recent memory.
U.S. Marines delivered drinking water and chlorine tablets to hundreds in Mapou, where officials have confirmed 300 dead so far. The town's one health clinic was underwater, so some of the wounded were being flown out by helicopter for treatment.
U.N. teams were trying to arrange boats to help recover bodies trapped under trees and in houses.
"You can still see bodies in the water coming up," said Michel Matera, a U.N. technical adviser for disasters who traveled to Mapou on Thursday.
If workers can't recover corpses soon, he warned that could contaminate water sources. "There is a grave risk of an epidemic."
An estimated 10,000 people in 26 villages surrounding Mapou are in urgent need of help, Matera said.
The floods struck early Monday following three days of heavy rains, triggering torrents that swept away entire neighborhoods.
One 18-year-old Haitian, Pepe Dematin, traveled across the border to Jimani searching for his brother's family of five. "I came to find them, but their house is gone," he said.
Dominican authorities buried more than 275 bodies after the floods struck early Monday, some in a mass grave. Officials said they plan to spray disinfectant by plane over the Dominican border town of Jimani to keep decomposing bodies from spreading disease.
Red Cross workers helped search for bodies, survey the damage and treat the wounded.
In the Haitian town of Fond Verrettes, a team from France-based Doctors Without Borders treated villagers with injuries. Matera said the World Food Program distributed 13 tons of food aid in the town.
Forecasts of more rain for the southern border region raised fears of more flooding. "There is the urgent need to move people away from some areas," Matera said.
The U.S.-led force, brought in to help after the Feb. 29 ouster of President Jean-Bertrand Aristide, struggled to fill urgent needs while they prepared to hand over control to a U.N. force on Tuesday.
Haiti has become a particularly hazardous place for flooding and mudslides because its impoverished people constantly fell trees to make charcoal, leaving the country almost entirely deforested. Without roots to hold back the soil, rains can bring disaster.
"It's a country where people see so much suffering anyway and particularly this year. They've just been smacked with political problems, economic problems," said Bracken.
The floods were some of the deadliest in the region in recent years. In 1994, Tropical Storm Gordon caused mudslides that buried at least 829 Haitians. As many as 15,000 people are estimated to have died in 1999 flooding and mudslides in Venezuela.
Touring Jimani on Thursday, Dominican President Hipolito Mejia promised homes for families who had lost them.
Interim Haitian President Boniface Alexandre called for flags to be flown at half-staff Friday and said he would ask the government to free up $250,000 in aid. The government, however, has scant resources to deal with natural disasters.