Aid workers have expressed shock at the scale of the disaster
People in the Haitian village of Mapou, where about 300 bodies have been found, spoke of clinging to trees for hours, then finding they had lost everything.
International aid, including food, clean water and first-aid kits, is beginning to reach the worst-hit areas.
But more than 900 people are known to have died, with hundreds more missing.
Rain is still falling, and more is forecast for the coming days.
In Mapou, thousands of people have sought shelter on the plateau above the village which remains almost entirely under water.
Jean-Claude Germain, 25, a farmer, says he lost four of his children.
The magnitude… is much worse than we expected with many, many more people affected
UN World Food Programme
“I had to watch everything I love and own washed away by the waters, but I never saw my children being taken,” he told AP news agency.
Ivse Toussaint, 35 says his wife and six children, aged two to 16, died in the deluge.
“I tried to get my kids up on the roof but the water was moving too fast,” he said.
Aid workers have expressed shock at the scale of the disaster.
“The magnitude… is much worse than we expected with many, many more people affected,” Guy Gavreau, director of the UN World Food Programme in Haiti, told AP.
“It’s horrific,” said Sheyla Biamby of Catholic Relief Services in Port-au-Prince.
“People are finding people in very odd and unreachable places – even hanging from the tops of trees.”
She said half the homes in Mapou – about 1,300 – had been destroyed.
The race is on to recover corpses floating in the water to stop disease spreading throughout the region.
Helicopters have sprayed the Dominican Republic town of Jimani, near the border with Haiti, with disinfectant.
Marko Kokic of the International Federation of Red Cross and Red Crescent Societies told the BBC that aid workers were trying to get hold of small boats so that volunteers could help recover the bodies.
“It poses a grave health risk for the population there,” he told the World Today programme.
Another key priority, Mr Kokic said, was to move the survivors out of the danger zones in case there was more flooding, and to send in water purification tablets, shovels, and body bags.
The flash floods have destroyed homes and washed away roads in towns and villages across the two countries which make up the Caribbean island of Hispaniola.
Two teams of United Nations disaster relief experts arrived on the island on Friday to help local UN relief teams and other aid workers.
The grim task of collecting the dead continues in Jimani
“They are there to do a rapid evaluation of the situation and the needs on the ground,” said Elisabeth Byrs, a Geneva-based spokeswoman for the UN Office for the Co-ordination of Humanitarian Affairs.
“We believe there are more than a total of 48,000 people affected in the two countries.”
The UN’s World Food Programme is flying 8,000 tonnes of aid into the region.
US-led peacekeeping forces – who were sent to Haiti to help keep order after the February overthrow of President Jean-Bertrand Aristide – have been helping the relief effort.
Rivers of mud
Many flood-hit towns are impossible to reach except from the air. And continuing rain has hampered the access even of helicopters.
Even before the flooding, Mapou was said to take three or four hours to reach from Jacmel, the nearest city.
In Jimani, rescuers continued to dig through rivers of mud, reported AFP.
The town – now a muddy scar in the landscape, after the flood waters drove through – was the worst-hit in the Dominican Republic.
At least 329 bodies have been found there, and at least 300 people are missing.
Dominican President Hipolito Mejia and the US ambassador, Hans Hatler, have toured the stricken town.
Mr Hatler announced $50,000 in aid, saying more would be delivered as soon as possible.