The United Nations and other aid agencies tried to ferry water and chlorine tables to the worst hit towns on Friday but bad weather is holding up efforts.
A spokeswoman for the United Nations' Office for the Coordination of Humanitarian Affairs said there are 1,500 dead or missing, and warned that this toll could rise.
Forecasters predicted more rain in the coming days as US Marines hurried on Friday to deliver drinking water and other supplies to hundreds in Mapou, where 1,000 were feared dead.
Some 450 corpses have been recovered in Haiti and 415 have been found in the Dominican Republic.
At least 1,000 were feared dead in the town, submerged in more than three metres of water, according to local government representatives. If the toll is correct in Mapou, the overall toll could be nearly 2,000.
Mudslides have washed out roads in southern Haiti, preventing workers from getting an accurate toll. More than 300 bodies alone have been found in Mapou, about 50 km southeast of the Haitian capital of Port-au-Prince.
United Nations teams planned to bring in boats to help recover bodies. If workers can't recover the corpses in time, they could contaminate water sources.
"You can still see bodies in the water coming up," said Michel Matera, a UN technical advisor for disaster and risk management who traveled to Mapou. "Palm trees are almost covered. There is a grave risk of an epidemic."
An estimated 10,000 people in 26 surrounding villages are in urgent need of help and essentially cut off by roads devoured in the mud and landslides, said Matera.
For many Haitians, it was one more disaster to pile atop the troubles weighing down on Haiti, the poorest country in the Americas.
Red Cross, UN and multinational
forces struggle to help survivors
The US-led force, brought in to help after rebels ousted President Jean-Bertrand Aristide, struggled to fill urgent needs while they prepared to handover control to a UN force on Tuesday.
The hurricane season, which marks the beginning of the rainy season, also starts on Tuesday.
Rains over the weekend lashed the island of Hispaniola, which Haiti shares with the Dominican Republic, sweeping away entire neighborhoods early on Monday.
In the Haitian border town of Fond Verrettes, troops on Thursday helped villagers by delivering plastic tarpaulins for shelter.
Rushing waters and mudslides swept away most homes in Fond Verrettes, leaving it looking like a barren riverbed. About 160 people were still missing and presumed dead there.
Fear of diseases
Hundreds are missing in both Haiti and the Dominican Republic.
American and Dutch Red Cross workers were helping Dominican authorities search for more bodies and treat dozens who were wounded, said Gustavo Lara, of the Dominican Red Cross. Workers were putting up mosquito nets to prevent mosquito-borne malaria and dengue fever.
The floods struck before dawn on Monday while people were asleep. Some watched relatives and homes carried away in torrents of mud.
Dominican authorities buried more than 250 bodies immediately, some where they were found and others in a mass grave. Authorities told families there was no time to identify the bodies because they were badly decomposed.
Haiti has become a hazardous place for flooding and mudslides because its impoverished people constantly fell trees to make charcoal, which has left the country almost entirely deforested. Without roots to hold back the soil, rains can bring disaster.