Angry Haitians looted United Nations aid trucks shortly before the UN secretary general visited parts of the country hit by Hurricane Matthew, which killed about 1,000 people.
Clashes were reported in Les Cayes, where Ban Ki-moon said "all resources" would be mobilised to help the nation.
Haiti says more than 1.4 million people urgently need humanitarian aid.
There are also fears of a cholera outbreak, with spikes in cases and deaths reported in the south west.
The waterborne disease reached the island via Nepalese UN troops after the deadly earthquake in 2010, causing the deaths of nearly 10,000 people.
Tensions have been high as help has yet to reach many families whose crops and water supplies have been destroyed.
Some 120,000 homes were either damaged or destroyed when Category Four Hurricane Matthew swept through on 4 October with winds of 145mph (230km/h).
Ban was greeted by Prime Minister Enex Jean-Charles before both men boarded a helicopter to fly over the south to inspect the damage caused by the storm.
"I was very, very sad when we saw the complete devastation. But people the world over stand with you," he said.
"The United Nations stands by your side. We will mobilise all resources to help you."
He visited a shelter in Les Cayes and told the displaced "kembe fem" in Creole, meaning "hang in there".
In the town, one of the worst affected by Matthew, the streets have been cleaned and businesses have reopened.
But power, transport, and communications links are still difficult in other areas.
Last week, Ban called for a "massive response" to help the country, the poorest in the Americas.
The UN launched an emergency appeal for nearly $120 million in aid, but just a fraction of it has been raised; according to agencies.