Florida health officials said they believe that four people have been infected locally with Zika, in what would be the first evidence that mosquitoes carrying the virus are present in the continental United States.
Officials have been investigating four cases of Zika in southern Florida that were not thought to be linked to travel to affected regions outside the country.
It is believed that the active transmission is happening in one small area in north Miami.
Governor Rick Scott told a news conference that one of the cases involved a woman, and the other three men.
Officials had been testing mosquitoes in the small area of southern Florida where the cases are located "for about two weeks," he added.
"While no mosquito traps have tested positive for the Zika virus, the Department of Health is aggressively testing people in the affected area to ensure there are no other cases of this virus," he said.
The health department had urged residents and visitors to participate if asked for urine samples in the areas being investigated, to help determine the number of people affected.
Zika is spread via mosquitoes and by sexual contact. Pregnant women who are infected face a higher risk of bearing an infant with microcephaly, a birth defect that causes an abnormally small head.
Florida has already seen almost 400 cases of Zika, all involving people who were infected while travelling to parts of the world where the virus is circulating.
For Zika to become a homegrown virus in the mainland United States, a mosquito would have to bite a Zika-infected person and then bite another person, passing on the virus.
Health officials have warned of possible localized Zika outbreaks in the United States, particularly since the virus has spread quickly throughout Central and South America and the Caribbean in the past two years.
However, mosquito control measures such as air conditioning, use of window and door screens and bug repellant are likely to prevent Zika from becoming established in the United States.
Zika virus detected in three patients in UK who had been overseas
Meanwhile, three people have tested positive for the Zika virus after returning to the UK from abroad.
The Calderdale and Huddersfield NHS Trust said that the patients were carrying the infection but released no further information.
A report from the trust's director of infection prevention and control said these cases were identified in the period 1 April to 30 June.
Infection control lead for the trust Dr Gavin Boyd, said: "There is extremely low risk of contracting Zika virus in the UK as the mosquito that transmits the virus is not present in the UK.
"A small number of cases of sexual transmission globally have been reported but the risk is very, very small.
"There is no specific treatment and it wears off naturally after two to seven days. After a diagnosis patients are cared for by their GPs."
A total of 53 people have now been treated in the UK for the Zika infection.
Since the Zika epidemic began in 2015, nearly 5,000 cases of microcephaly have been recorded in affected regions.
On 1 February this year the World Health Organization declared the epidemic an international public health emergency.
Worldwide concern has centred on north-east Brazil but more than 20 other countries have now been affected.
The mosquitos that transmit the infection are now thought to be able to live in colder climates.
Pregnant women have been advised not to travel to the countries affected by Zika and a number of prominent sports stars have pulled of the Olympic Games in Rio as a result.
