Hedi Annabi says an exceptional effort is needed to help Haiti's storm victims
The UN special envoy to Haiti says the country has been overwhelmed by the effects of recent storms.
Hedi Annabi said the scale of the disaster was beyond what the UN and Haitian authorities could deal with and more international help was needed.
He said the western city of Gonaives was all but destroyed and every house had been damaged or swamped by mud.
He suggested Haiti's gross domestic product had fallen by between 3-4% because of the storms.
Haiti was hit in the space of a month by tropical storms Fay and Hanna and hurricanes Gustav and Ike, leaving about 550 people dead.
Prime Minister Michele Pierre-Louis has said that up to one million Haitians may be homeless.
Plea for donors
"This is a humanitarian catastrophe of a scale that is beyond the capacity of the government, [or] of the UN stabilisation mission here," said Mr Annabi.
"It requires an exceptional effort from bilateral donors, from those countries in the region, or beyond, that have the kind of assets that are required to deal with such an emergency."
Mr Annabi said Gonaives was completely flooded.
The city of Gonaives has been left in ruins by recent storms
"There is not a single house that has not been destroyed or damaged, that is not full of two or three feet of mud," he said.
"It is just an horrendous sight. This is a city which is basically destroyed."
UN officials say the World Food Program has distributed food to more than 230,000 people in Gonaives.
But about 800,000 people are in dire need of help, it says.
Sarah Wilson, of the Christian Aid assessment team in Haiti, told the BBC that more food and medical supplies were getting into Gonaives since the road to the city was repaired.
"Food is now distributed twice a day but shelter remains a big problem," she said.
"Schools are due to reopen in early October, yet thousands of homeless people are still living inside school buildings because their homes have either been washed away or are uninhabitable.
"It is not just short term emergency relief that people need. Without seeds to replant or livestock to replace those that were killed, hundreds of thousands of people in Haiti will have no way to feed themselves for months to come.
"Christian Aid is helping people to restock, but more help from international donors and the general public is badly needed if we are to complete the task."
Mr Annabi added that storm damage in the agricultural sector alone amounted to more than $200m (£111m), according to an incomplete assessment.
Earlier this month the UN launched an appeal for $108m (£60m) to help the storm-battered nation. Many of Haiti's nearly nine million population live on less than $2 a day.