Pregnant women have been warned to avoid the international tourist destination Miami Beach, amid cases of the Zika virus.
Authorities had previously thought the virus was limited to Wynwood, a small area west of Miami Beach.
Health officials now say five people in Miami Beach have been infected. Florida health officials have been aggressively spraying pesticides there.
Zika, often spread by mosquitoes, can cause life-threatening birth defects.
"We're in the midst of mosquito season and expect more Zika infections in the days and months to come," Tom Frieden from the Centers for Disease Control and Prevention (CDC) said on Friday.
Florida has now had 36 cases of the disease, not counting those that were connected to travel outside the US.
Known for its Art Deco architecture and expensive shops, Miami Beach attracts millions of tourists each year.
Critics have said that Florida Governor Rick Scott has been delaying the release of information and downplaying the threat of Zika to protect the state's billion-dollar tourism industry.
But Governor Scott said the state was taking every measure to ensure the information they provided to the public was accurate.
"We recognise the desire for information quickly, but it is important that we conduct our interviews and investigations pursuant to epidemiological standards," he said.
Senator Harry Reid, the Democratic minority leader, said the transmission of Zika in Miami Beach "is the most alarming development yet in the rapidly growing threat of Zika in the United States".
Senator Reid along with Governor Scott urged Congress to provide additional funding to the area. Democrats and Republicans in Congress have been at odds for months over a bill that would contribute to the aid efforts in Florida.