"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).
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.