The devastating hurricane wiped some towns and villages off the map, the UN chief said, after pledging $120m in aid.
Haiti faces a crisis that requires a "massive response" from the international community, the United Nations has said, with at least 1.4 million people needing emergency aid after Hurricane Matthew.
The storm killed almost 1,000 people in the impoverished Caribbean nation, with that toll likely to rise as rescue workers reach previously inaccessible areas.
Matthew, the most powerful Atlantic storm since 2007, last week leveled homes, fouled water sources and killed livestock, leaving victims pleading for help to arrive quickly.
"Some towns and villages have been almost wiped off the map," UN Secretary-General Ban Ki-moon told reporters.
The UN has launched a $120m flash appeal to cover Haiti's needs for the next three months.
After pummeling Haiti on October 4 as a monster Category 4 storm, packing winds of 230km an hour, Matthew slammed into the southeastern United States, where it killed at least 20 people.
Hundreds of people were rescued by boat and helicopter as floodwaters inundated towns in the state of North Carolina on Monday, and officials warned that life-threatening flooding from swollen rivers would continue for days.
Memories of 2010 quake
In Haiti, more than 300 schools have been damaged, while crops and food reserves were destroyed, Ban said.
UN aid chief Stephen O'Brien said the hurricane had triggered the worst humanitarian crisis in the country since the 2010 earthquake.
Nearly 300,000 Haitians are in shelters across the country, and damage to roads and communications has hampered deliveries of supplies.
"I understand of course the frustration," Jean-Luc Poncelet, the country representative for the UN's World Health Organization, said after arriving at the airport outside Jeremie, one of the worst-hit cities.
"When you have no means of communication, no radio, no telephone, no roads and even a helicopter can't land - this is what explains the massive delay," he told the AFP news agency.
The UN's World Food Programme tapped into food stocks previously set aside for schools to feed hundreds of desperate families, spokesman Alexis Masciarelli said.
US army helicopters were unloading boxes of supplies from the US Agency for International Development to be stored by the UN in Jeremie before being taken to other parts of the south.
Honduras, which maintains a force of 60 troops in Haiti as part of a UN peacekeeping mission, was sending a planeload of aid on Tuesday, along with 50 military officers to help the victims, President Juan Orlando Hernandez said.
But getting aid to Haitians now reduced to drinking unclean water and living in roofless houses will be challenging.
On a road crossing the mountainous centre of the peninsula, some villagers blocked roads in an effort to stop aid convoys from passing through without delivering supplies.
Haiti is also grappling with a worsening cholera outbreak in the storm-hit areas.
Matthew came as Haitians were already struggling with the intestinal disease spread by contaminated food and water, with more than 500 new cases each week.
UN peacekeepers have been blamed for introducing the disease to Haiti, where it has killed 10,000 people since October 2010.
While some towns and villages reported an apparent spike in infections since the storm, Poncelet said "the number of cases of cholera that we have confirmed are low".
He declined to give a number, but said there were "tens" of cases in one area of the peninsula.
While evaluation teams were working to get a precise picture of the health situation, medical supplies were being brought in, he said.
Mourad Wahba, the UN humanitarian coordinator in the country, urged aid organisations to focus on delivering supplies to smaller rural communities, where many families survive on subsistence farming and have had all their crops washed away.
If aid is only delivered to cities such as Jeremie and Les Cayes, villagers will flock there for supplies and never leave, leading to overcrowding.
"We must think about developing a plan, to coordinate support and deliver it where it's most needed and not where it's easiest to access," Wahba said.
The storm has triggered the worst humanitarian crisis in Haiti since the 2010 earthquake, as hundreds of thousands reside in shelters