Haiti's president has appealed for aid to help the island in the aftermath of tropical storm Jeanne whose floods have claimed nearly 700 lives.
Floods devastated Haiti's northwest
Boniface Alexandre told a meeting of the United Nations General Assembly in New York that the storm had left a humanitarian disaster in its wake.
Large swathes of the northwest of the country remain submerged and some 1,000 people are still missing.
The northern port city of Gonaives was the worst-hit by rising floodwaters.
Haiti's interim Prime Minister Gerard Latortue told the BBC that more than 80,000 people in the northwest of the Caribbean country had absolutely nothing to eat.
UN peacekeeping troops - sent after the overthrow of President Jean-Bertrand Aristide - are already working with aid agencies to help the victims of the storm.
Haiti - the poorest country in the Western hemisphere - is particularly vulnerable to tropical storms, correspondents say.
Heavy deforestation has left few trees to hold back water and low lying areas have no protection from heavy rains.
"In the face of this tragedy, which is of the magnitude of a humanitarian disaster, I appeal urgently for the solidarity of the international community," Mr Alexandre said.
Officials said some 600 bodies had been retrieved in Gonaives alone and around 100 bodies elsewhere.
Poorest country in Western Hemisphere
80% live below absolute poverty threshold
Severe or moderate stunting affects 47% of under fives
*Data from UN World Food Programme
"We are going to start burying people in mass graves later Tuesday," UN spokesman Toussaint Kongo-Doudou said.
Aid workers warned the number of dead was likely to increase as the waters subsided.
"I'm afraid those figures are going to increase, as there are areas that still remain inaccessible," Hans Havik of the International Federation of Red Cross Societies told Agence France Press.
Residents forced to take shelter on rooftops reported bloated bodies floating along the flooded streets of Gonaives.
Days of torrential rain have sparked mudslides, isolating communities as roads were destroyed and homes swept away by the water.
"The river destroyed my house completely and now we have nothing," 18-year-old Katya Silme told the Associated Press news agency, adding she and her family spent the night in a tree.
Initial reports suggested the island of La Tortue, off the northern coast of Haiti, had been completely submerged by the rising water levels.
Mr Latortue earlier described the flooded area as "a vast sea" and declared three days of national mourning.
The UN World Food Programme (WFP) estimates 175,000 people are without food, water and electricity and in need of help.
"The floodwaters were so strong in Gonaives that they have washed away the whole town," WFP Country Director Guy Gauvreau told BBC News Online.
"People don't have the means to cook, and they lack water."
"We could have had an earthquake or a civil war but this is worse because it has washed everything away," he said.
In the past year Haiti has faced a series of crises including armed revolts, military intervention and deadly floods.
Aid efforts led by international agencies are under way, assisted by 3,000 UN peacekeeping troops already in the country.
But severed road links and a tense security situation are hampering efforts.
The WFP said aid trucks carrying emergency food supplies had been lined up to form a makeshift bridge over the water.
The International Red Cross said it was considering the use of boats to reach trapped people.
The EU announced it would send 1.5m euros ($1.8m) in urgent aid, and the US Embassy offered $60,000 in immediate relief.
NORTHERN HAITI UNDER WATER
1. The island of La Tortue was barely visible but Haitian officials now say nothing happened there.
2. About 30% of Port-de-Paix city is reported to be covered by water along the coast and nearby plantations are reported to be under water. The road to Gonaives is flooded.
3. The water level of the river is reported to be very high and some villages on its banks have been flooded.
4. Flooding has hit all urban areas of Gonaives, affecting about 175,000 people - some 600 people are believed to have died there.