US health officials are recommending pregnant women 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 said local mosquito control efforts have not worked as well as hoped.
It said pregnant women who live or work in the mixed-use area north of downtown Miami and their partners should make every effort to avoid mosquito bites.
The CDC said people returning from the affected area of Florida should use mosquito repellent for three weeks to protect their families and guard against further transmission at home.
It also recommended that women avoid getting pregnant for up to eight weeks after returning from the affected area.
The agency said that pregnant women who live in or travelled to the affected area after June 15 be tested for Zika.
The focus on pregnancy stems from that fact that Zika has been found to cause microcephaly and other potentially devastating birth defects.
Florida now has 14 cases of Zika caused by locally transmitted mosquitoes, according to a statement issued on Monday 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 the microcephaly, and has since spread rapidly through the Americas.
Scott said the state has 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 by Tuesday.
"Nothing we have seen suggests widespread Zika virus transmission," 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 goes 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.
Hotez believes there are other likely local outbreaks occurring and that more can be expected in the next six weeks in Florida and other Gulf Coast states where the mosquito that carries the virus is common.
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