Zika fears prompted US health authorities yesterday to issue a travel warning for a small section of Miami where local mosquitoes have spread the virus to 14 people, officials said.
"We advise pregnant women to avoid travel to this area," said Centers for Disease Control and Prevention chief Tom Frieden, noting that the virus can cause the birth defect microcephaly.
The area to avoid is inside a 1.6km section north of downtown Miami, a popular arts and restaurant district known as Wynwood.
Women who are pregnant and live in or may have traveled to the area since June 15 should talk with their doctor, Dr Frieden added.
Pregnant women in the area are also urged to use barrier protection during sex, or to abstain in order to lower the risk of transmission from a partner.
He also recommended people use mosquito repellent, wear long sleeves, repair screens and drain any standing water to prevent the spread of the mosquitoes.
"In Miami, aggressive mosquito control measures don't seem to be working as well as we would have liked," said Dr Frieden.
He said it was possible that mosquitoes are resistant to insecticides currently being used, or that they may have hidden breeding areas that haven't been found yet, or that this type of mosquito - the Aedes aegypti - is simply difficult to control.
Dr Frieden said most people with Zika do not show any symptoms.
"Nothing that we have seen indicates widespread transmission but it is certainly possible there could be sustained transmission in small areas."
On Friday, Florida officials announced the first locally transmitted cases of Zika in the United States with all four linked to the same area in Miami.
Early Monday, Governor Rick Scott said the number of identified cases had jumped by 10 to 14.
The cases mark the first time the Zika virus, which can cause birth defects and is considered particularly dangerous for pregnant women, is known to be spreading via local mosquitoes in the United States.
Over 1,600 cases of Zika have been previously reported in the US, but most were brought by travelers who were infected elsewhere. The virus can also spread by sexual contact.
The CDC is sending an emergency team of specialists to help the Florida response, Dr Frieden said.
Two of the 14 cases involve women and the rest are men. At least six were not showing any symptoms but were identified during door-to-door surveys and testing.
Dr Frieden said the decision to issue a travel warning is an unusual measure for the continental United States.
"We can find no similar recommendation in recent years," he told reporters on a conference call.
A travel warning for the US territory of Puerto Rico was issued in January when Zika began to circulate there.
According to the World Health Organization, 67 countries and territories have reported mosquito-borne Zika virus transmission since 2015.
Brazil has been particularly hard hit, with more than 1,700 babies born with unusually small heads, a key feature of microcephaly.
Dr Frieden said that each child born with microcephaly can require $10 million in medical costs over a lifetime, but convincing people to take strong measures against Zika can nevertheless be a challenge.
"The tragedy of a preventable case of a severe birth defect is something I think we have to make very clear to people," he said.
"It is truly a scary situation but it is not immediately apparent to people that it is this kind of significant risk."
Funding for the Zika response has also been a source of dispute among US lawmakers.
US President Barack Obama asked for $1.9 billion in February, but Republicans protested, saying the money should be taken from funds previously set aside for Ebola. Congress went on summer recess last month without approving any legislation for Zika funds.