Up to 400 people are now believed to have died in Haiti from flooding and mudslides triggered by Tropical Storm Jeanne.
Aid workers say half of the northern city of Gonaives is still underwater.
"We have counted 250 bodies at the hospital morgue in Gonaives," UN spokesman Mr Toussaint Congo-Doudou said after heavy rains sent a wall of muddy water crashing through northern towns over the weekend.
UN peacekeepers had unconfirmed reports of another 150 dead in Gonaives, said UN coordinator Mr Adama Guindo.
The northern city was the birthplace of Haiti's independence from France 200 years ago and it was where an armed revolt began this year that led to the ouster of President Jean-Bertrand Aristide.
Forty-seven people were also confirmed killed in the northwest province, around the town of Port-de-Paix, said a government official. Eight deaths were recorded elsewhere, bringing the total confirmed toll to 305.
Interim Prime Minister Mr Gerard Latortue declared three days of national mourning.
Half of Gonaives remained under water, and 80 per cent of its inner urban population of over 100,000 had been affected by the floods, which at one point forced hundreds of people to take cover on the roofs of their homes, said Ms Anne Poulsen, spokeswoman for the UN's World Food Programme in Haiti.
Twelve trucks carrying 40 metric tons of food would leave the capital Port-au-Prince Monday and head to Gonaives, said Ms Poulsen.
The World Health Organization planned to deliver medicine, and 15 trucks from the Brazilian-led UN force had gone to reinforce a detachment of Argentine peacekeepers stationed in the city.
"It's not just people's houses, it's also crops and livestock that have been washed away. So it will take quite some months before people will be able to cope by themselves again," Ms Poulsen said. "Nature is tough on Haiti."
Haiti, the poorest country in the Americas, is frequently lashed by flash floods and mudslides because of extensive deforestation. Around 2,000 Haitians died when extensive floods washed away villages near the Dominican-Haitian border in May.