US officials have issued a new advisory that says pregnant women should not travel to the so-called Zika "transmission area" in Florida.
The Centres for Disease Control and Prevention also said that pregnant women who live there should take steps to prevent mosquito bites and sexual spread of the virus.
The CDC issued the advisory after Florida Governor Rick Scott said there are 10 new infections of the Zika virus likely transmitted by mosquitoes, bringing the total in the state to 14.
The new cases are clustered in the same square-mile neighbourhood in Miami-Dade County identified last week.
Florida health officials say they believe active transmissions of Zika are occurring only in that area.
The CDC says men and women who have visited this area since June 15 should wait at least eight weeks before trying to conceive.
Because Zika infection has been found to linger in sperm for months, men with Zika symptoms should wait at least six months before trying to have a baby with their partner.
Zika infections in pregnant women can cause severe brain-related birth defects. The outbreak has led to more than 1,800 cases of microcephaly, in which a baby is born with an extremely small head.
US health officials have said all along that while isolated clusters of Zika may occur in this country, they do not expect major outbreaks like those seen in Latin America, because of better sanitation and mosquito control and widespread use of air conditioners and window screens.
CDC officials said they could not remember another time in the 70-year history of the agency when it told members of the public not to travel to a certain place within the United States.
The travel warning covers an area of about one square mile in Wynwood to the east of Interstate 95 and south of I-195.
It is large enough, health officials said, to provide a buffer around the suspected hot zone. The tropical mosquito that spreads Zika travels less than 200 yards in its lifetime.
Some experts said that is far too small a radius. Dr. Peter Hotez, a tropical medicine expert at the Baylor College of Medicine in Texas, said the CDC should be more cautious and expand the travel advisory to all of Miami-Dade County.
"If you're pregnant or think you might be pregnant, avoid travel to Miami, and possible elsewhere in South Florida," he said.
"I'm guessing most women who are pregnant are doing that. I don't think they're sitting around for the CDC to split hairs and fine-tune it to a specific area."
CDC director Dr Tom Frieden said the narrowly drawn warning was dictated by science and not by any concern for Florida's crucial tourism industry.
He said it was based on the nation's ability to contain outbreaks of other diseases carried by the same mosquito.
"There wouldn't be a technical or scientific basis to give a broader recommendation," Dr Frieden said.
Florida health officials said they have tested more than 200 people in Miami-Dade and Broward Counties since early July. The CDC emergency team will help Florida officials investigate the outbreak, collect samples and control mosquitoes.
Of the 14 people infected, two are women and 12 are men. Eight patients showed symptoms of Zika, which can include fever, rash, joint pain and red eyes. The others had no symptoms. The disease is often so mild that most people do not know they are infected.
"We will continue to keep our residents and visitors safe utilizing constant surveillance and aggressive strategies, such as increased mosquito spraying, that have allowed our state to fight similar viruses," the governor said in a statement.