Jocelerme Privert said famine could take hold within three to four months if the situation was not managed properly.
It comes as UN Secretary General Ban Ki-moon called for a "massive response" to help the country.
The category-four storm is believed to have killed as many as 900 Haitians.
It has also wiped towns and villages off the map, destroying tens of thousands of homes, crops and food reserves.
Mr Privert said the loss was "amazing", saying food, water and medicine was immediately needed.