The Zika virus, believed to be linked to the serious birth defect microcephaly, presents a "formidable" challenge that will be hard to stamp out, according to World Health Organization chief Margaret Chan.
Ms Chan was in Rio de Janeiro on a fact-finding mission, where she said "Things could get worse before it gets better”.
Ms Chan said part of the challenge in fighting the mosquito-borne virus was the fact that it is so "mysterious." Even the link to microcephaly remains not fully proven.
"We are dealing with a tricky virus, full of uncertainties, so we should be prepared for surprises," she said.
Ms Chan praised the Brazilian government's efforts to stamp out mosquitoes and its coordination with various international bodies, including the WHO and the International Olympic Movement, ahead of the Olympic Games in Rio this August.
Brazil is the main focus of a Zika outbreak, with 1.5 million people infected, and authorities have also recorded a spike in microcephaly, a congenital condition that causes abnormally small heads and hampers brain development.
On Tuesday, Brazil's health ministry reported 583 confirmed cases of babies with microcephaly since October, compared to an annual average of 150.
That was a 14.7% rise over the number of confirmed cases the previous week, and authorities were investigating another 4,107 possible cases.
An estimated 120 babies have died due to the birth defect, the ministry said.