Thousands of people in Central and South America and the Caribbean are infected with the Zika virus.
Authorities have confirmed the first sign of local Zika transmission in the continental US, concluding that mosquitoes most probably infected four people with the virus.
Governor Rick Scott said on Friday that the state believed active transmission of the virus was occurring within an area of Miami about the size of square mile [2.6sq km].
Testing showed that one woman and three men had been infected.
While health officials have yet to identify mosquitoes carrying the virus, the state has ruled out other means of transmission, including travel to another country with a Zika outbreak and sexual contact.
“This means Florida has become the first state in the nation to have local transmission of the Zika virus,” Scott said.
Zika appears to pose the greatest risk when it infects pregnant women, given its ability to cause rare but serious birth defects.
The local health department is searching for other potential infections, with more than 2,300 people tested so far in the state.
It is strengthening mosquito-control programmes and is distributing Zika protection kits to pregnant women at their doctors’ offices, Florida officials said.
However, Florida has yet to invite a dedicated team of the federal government’s disease hunters to assist with the investigation on the ground, health officials told Reuters news agency on Friday.
Coordination with the US Centers for Disease Control and Prevention (CDC) has been conducted largely at a distance, they said.
Some infectious disease experts say a less robust response could lead to a higher number of infections.
Other states have quickly called in CDC teams to help track major diseases.
“You only have a small window. This is the window” to prevent a small-scale outbreak from spreading, said Dr Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston, who expressed impatience with the pace of the Florida investigation.
Scott said the state health department was working with the CDC as it continues its Zika investigation.
CDC said it is closely coordinating with Florida officials who are leading the effort. Dr Marc Fischer, a CDC epidemiologist, has gone to Florida at the state’s request.
But the state has not invited in the CDC’s wider emergency response team of experts in epidemiology, risk communication, vector control and logistics, according to Mara Gambineri, Florida health department spokeswoman.
In its plans to fight Zika nationwide, CDC stressed that such teams would help local officials track and contain the virus.
“Should we need additional assistance, we will reach out,” Gambineri said in an email to the Reuters News Agency. She did not reply to questions about why the state decided not to bring in a CDC team.
Until now, more than 1,600 Zika cases in the US have resulted from travel to another country with active transmission, as well as a small number of cases of apparent sexual transmission by a person infected outside the country.
Puerto Rico, the island territory, has seen several thousand confirmed cases of local transmission.
US health officials have predicted there will be hundreds of thousands of cases on in Puerto Rico before the current outbreak ends owing to the prevalence of Zika-carrying mosquitoes and a lack of infrastructure to protect against insect bites.