Pregnant women are being advised by Public Health England to consider postponing non-essential trips to the US state of Florida, amid concerns over the Zika virus.
Its travel advice was updated after Florida confirmed four cases of the virus transmitted by local mosquitoes.
Previous US cases have been connected to people who caught the virus abroad.
Zika causes only a mild illness in most people, but the virus has been linked to severe brain defects in newborns.
The US Center for Disease Control does not expect a widespread outbreak of Zika in the country, but says it is preparing for small clusters of infections.
Florida has announced more aggressive mosquito-control efforts, and politicians are keen to assure tourists the state is safe to visit.
Paul Cosford, medical director and director of health protection at Public Health England, said the risk in Florida was considered "moderate", based on the number of cases and control measures in place.
"Pregnant women are advised to consider postponing non-essential travel until after pregnancy.
"Advice to all travellers remains to avoid mosquito bites."
He said Public Health England was monitoring the international situation closely.
The advice from Public Health England is based on a list of countries and territories with current active Zika virus transmission as classified by the European Centre for Disease Control.
Florida is listed as having a moderate risk rating for the Zika virus, along with Guinea-Bissau, Indonesia and the Philippines.
Many more areas, including a number of countries in south and central America and the Caribbean, are categorised as being "high" risk.
In those countries, the advice is for pregnant women to postpone non-essential travel, rather than just considering postponing.
Health Protection Scotland has also updated its advice for travellers following the confirmation of the Zika cases in Florida, while links to Public Health England's guidance are provided on the websites of Public Health Wales and Northern Ireland's Public Health Agency.
In terms of the UK, Professor Cosford said more than 50 cases had been diagnosed since January, but the risk to the UK remained unchanged.
He said a small number of Zika virus infections in travellers returning to the UK was to be expected, but the risk to the wider population was "very low" as the mosquito that spread the virus was not found in the UK.
He added: "If you have recently returned from an area where Zika virus transmissions are currently reported and have a fever or flu-like illness, seek medical attention without delay to exclude malaria and mention your recent travel."