The infected persons have "not exhibited symptoms" requiring medical attention, said Florida's governor. US health authorities have yet to issue a travel warning to the impacted area north of downtown Miami.
Florida state Governor Rick Scott on Friday said four people have been infected with the Zika virus, noting that authorities believe it is the first incident of local transmission from mosquitoes.
"They are all active Zika cases, and have not exhibited symptoms to be admitted to the hospital," Scott said in a statement.
"We have worked hard to stay ahead of the spread of Zika and prepare for the worst. We will continue to put every resource available to fighting the spread of Zika in our state," he added.
Florida's health department investigators determined that the four non-travel cases in Miami and its surrounding areas have a "high likelihood" to have been contracted locally.
The department added that the "active transmission of the Zika virus is occurring in one small area in Miami-Dade country," north of downtown.
Centers for Disease Control (CDC) Director Tom Frieden said authorities would not limit travel to the area, although he warned pregnant women to avoid mosquito bites.
"All the evidence we have seen indicates that this is mosquito-borne transmission that occurred several weeks ago in several blocks in Miami," Frieden said.
"We will continue to support Florida's efforts to investigate and respond to Zika and will reassess the situation and our recommendations on a daily basis," he added.
Meanwhile, White House deputy press secretary Eric Schultz said the federal government would provide assistance to the state to combat any spread of the virus.
US authorities in April announced the likely link between Zika and a rise in newborns with microcephaly, a rare condition resulting in babies born with small heads, which presents developmental challenges.
Brazil has been the hardest hit by the epidemic, with over 1.5 million infected and more than 1,000 cases of microcephaly registered since the outbreak in 2015.
ls/kms (Reuters, AFP, AP)