U.N. humanitarian coordinator Nigel Fisher told the BBC aid agencies must "ratchet up" their response and send additional medical staff.
"This epidemic is moving faster and we are in unknown territory in Haiti just because this is moving so fast," Fisher said. "There is no immunity to it."
He said more than 200,000 cases of cholera could be recorded in a three-month period rather than six months as initially estimated. The Haitian government says 1,415 people have died so far.
Fisher said the government must increase pressure on local authorities to open more treatment centers and burial sites.
The cholera epidemic has complicated preparations for elections scheduled for Sunday.