Pregnant women have been told to avoid travelling to a 2.6 square km area of Miami with active Zika virus transmission, after the state of Florida identified 10 more cases caused by local mosquitoes.
The US Centers for Disease Control and Prevention (CDC) told reporters that local mosquito control efforts had not worked as well as hoped.
Pregnant women who lived or worked in the mixed-use area north of downtown Miami, and their partners, should make every effort to avoid mosquito bites, it said.
People returning from the affected area were advised to use mosquito repellent for three weeks to protect their families and guard against further transmission at home, and women returning from the area were advised to avoid getting pregnant for up to eight weeks.
The agency also said pregnant women who lived in or travelled to the affected area after 15 June should be tested for Zika.
The virus has been found to cause microcephaly and other potentially devastating birth defects.
Florida now has 14 cases of Zika caused by local mosquitoes, according to a statement issued by Florida Governor Rick Scott.
The current Zika outbreak was first detected last year in Brazil, where it has been linked to more than 1700 cases of microcephaly, and has since spread rapidly through the Americas.
Mr Scott said the state had called on the CDC to activate a CDC Emergency Response Team (CERT) to assist the Florida Department of Health and other partners in their investigation, sample collection and mosquito control efforts.
CDC Director Dr Tom Frieden said a full eight-person CERT team should be in place within a day.
"Nothing we have seen suggests widespread Zika virus transmission," Dr Frieden said.
Florida had reported hundreds of cases of Zika in people who had travelled to areas with active outbreaks, such as Brazil and Puerto Rico. CDC said the first evidence of possible local Zika transmission in Florida went back to mid-June.
"To assume that it's just restricted to these few square blocks is presumptuous," said Dr Peter Hotez, dean for the National School of Tropical Medicine at Baylor College of Medicine in Houston.
Dr Hotez believed there were other likely local outbreaks occurring and more could be expected in the next six weeks in Florida and other Gulf Coast states, where the mosquito that carried the virus was common.
As of last Friday, Florida had not activated a CERT team to help with its investigation, raising concerns from infectious disease experts that the state was not taking every step it could to contain the spread of Zika in the continental United States.
The state said it began investigating its first suspected case of locally transmitted Zika on 7 July. According to CDC spokeswoman Kathy Harben, the CDC first became aware of the investigation on 18 July, a day before the state announced its investigation into possible local transmission.
Florida said on Friday the first four cases of Zika in the state were likely caused by mosquitoes, the first sign that the virus was circulating locally, although it was yet to identify mosquitoes carrying the disease.
Mr Scott said in a statement the 10 new cases of Zika were also likely caused by bites from local mosquitoes.
The Florida Department of Health said six of the 10 new cases were asymptomatic and were identified through its door-to-door community survey and testing.
The state health department has been testing individuals in three locations in the Miami-Dade and Broward counties for possible local transmissions through mosquito bites. Two locations had been ruled out for possible local transmission of Zika, it said.
Dr William Schaffner, infectious disease specialist at Vanderbilt Medical School in Nashville, said there was concern people infected in Florida would travel to other areas of the country where Zika could then be spread through local mosquitoes there.
Dr Schaffner and Dr Hotez said the government must come up with proper funding to fight Zika.
"Local and state health department budgets are very tight," Dr Schaffner said.