Thousands of people in Central and South America and the Caribbean are infected with the Zika virus.
Zika fears have prompted US health authorities 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,” Centers for Disease Control and Prevention chief Tom Frieden said on Monday, noting that the virus can cause the birth defect microcephaly.
The warning was issued for a one-mile section north of central Miami, a popular arts and restaurant district known as Wynwood.
Women who are pregnant and live in or may have travelled to the area since June 15 should talk with their doctor, 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 repellant, 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 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.
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.”
Al Jazeera’s Andy Gallacher, reporting from Miami, said: “The authorities here are concerned about people’s health but there could also be an impact on Florida’s tourism. It is the number-one industry here.
“But the overwhelming majority of experts here said they don’t expect Zika to spread [in the US] in the same way as we have seen in Brazil or Puerto Rico where the infrastructure isn’t quite as robust.”
On Friday, Florida officials announced the first locally transmitted cases of Zika in the US, with all four linked to the same area in Miami.
Early on 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 US.
More than 1,600 cases of Zika have been previously reported in the US, but most were brought by travellers 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, 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.
Frieden said the decision to issue a travel warning is an unusual measure for the continental US.
“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 first 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 hit particularly hard, with more than 1,700 babies born with unusually small heads, a key feature of microcephaly.