Ban Ki-moon said he witnessed a looting incident in Les Cayes, where he promised more aid to Haiti. He also urged countries to donate more.
Officials say more than 1.4 million people urgently need humanitarian help.
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.