Tropical Storm Hanna has caused widespread flooding in Haiti
Haiti faces a "catastrophe" after being hit by a series of storms in recent weeks, President Rene Preval has said.
Three storms in less than 21 days have killed more than 200 people, Haitian officials say.
A UN official says 600,000 people may need help because of flooding caused by Tropical Storm Hanna, which has caused the deaths of 136 people.
Mr Preval warned that Hanna could prove even more deadly than Hurricane Jeanne, which killed more than 3,000 in 2004.
Hanna swirled over Haiti for four days, dumping massive amounts of rain, blowing down fruit trees and swamping tin-roofed houses.
Massive need
The port city of Gonaives bore the brunt of the storm, with thousands of people seeking shelter on rooftops and balconies.
A team from the American Red Cross flew over Gonaives
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"There is no food, no water, no clothes," Arnaud Dumas, a pastor at a Gonaives church, told the Associated Press.
"I want to know what I'm supposed to do. We haven't found anything to eat in two, three days. Nothing at all."
The UN co-ordinator for humanitarian aid in Haiti, Joel Boutroue told the BBC: "We're already facing a lot of difficulty trying to respond.
"In Gonaives alone we have some 70,000 people in shelters, and around 250,000 around Gonaives City need our assistance and that of the government, and throughout the country I would say around up to 600,000 people might require our assistance."
An AP reporter in the city said safe drinking water was in very short supply, and fetid carcasses of drowned farm animals were strewn in soupy floodwaters.
Help was arriving in the area, with UN troops picking people from rooftops and Spain announcing that a planeload of aid was being flown in from Panama.
But floodwaters were frustrating efforts to distribute food, the UN said.
The British Red Cross announced it was launching an appeal, saying the needs of Haiti were "massive".
Red Cross workers were also helping residents of the Turks and Caicos Islands, north of Haiti, rebuild after Hanna ripped through there on Monday.
"Our volunteers have been supporting the shelters here with food and shelter management, transporting people to hospital, and handing out tarpaulins to help keep roofs on," said the organisation's Clive Evans, on the islands.
"There are abandoned cars everywhere, overturned boats, uprooted trees, downed power lines and flooded roads."
US prepares for storm
Earlier, Mr Preval said he would hold emergency talks with donor countries to appeal for aid.
Gonaives was battered with winds of 100km/h (65mph), leaving people on rooftops screaming for help as floods reached depths of 2m (6.5ft).
At 2100 GMT on Thursday, Hanna was about 250km (155 miles) east of Nassau and moving north-west, the US National Hurricane Center (NHC) said.
There are fears it could become a hurricane by the time it hits land along the US coast on Saturday, but the storm's uncertain path means officials are holding off ordering an evacuation.
However, a tropical storm watch has been issued from Virginia's border with North Carolina extending north to include Washington DC.
Some residents of North and South Carolina have already moved boats and booked inland hotel rooms.
Separately, storm Ike has strengthened rapidly into an extremely powerful Category Four hurricane in the open Atlantic, the NHC says.
However, it says it is too early to determine if Ike poses any threat to land.
Haiti was first drenched by Tropical Storm Fay, before Hurricane Gustav wreaked havoc last week, with torrential rainfall over heavily deforested and hilly terrain causing floods and mudslides.
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