The tropical storm, Jeanne, has brought floodwaters and mudslides to Haiti, killing at least 400 people.
Half of the northern city of Gonaives is still reported to be underwater. A UN spokesman said there were 250 bodies in the hospital morgue in the city with as many as 150 elsewhere.
Floodwaters tore down mountainsides and burst riverbanks following two days of steady rains.
The emergency services say providing aid is difficult because many roads are impassable and telephone lines have been severed.
The Haitian interim prime minister, Gerard Latortue, has appealed for immediate international assistance.
United Nations peacekeepers, who have been in Haiti since rebels overthrew the former president, Jean Bertrand Aristide, in February, have been deployed to help residents.
The storm had earlier caused extensive flooding and at least ten deaths in the neighbouring Dominican Republic.
