Florida authorities have reported the first sign of local Zika transmission in the continental US, concluding that mosquitoes likely infected four people with the virus that can cause a rare but serious birth defect.

Governor Rick Scott said the state believed active transmission of the virus was occurring within an area of Miami about the size of a square mile (2.6 square km). Testing showed that one woman and three men had been infected, Scott said.

While health officials have yet to identify mosquitoes carrying the virus, the state has ruled out other means of transmission, including travel to another country with a Zika outbreak, and sexual contact.

"This means Florida has become the first state in the nation to have local transmission of the Zika virus," Scott said at a press conference.

Zika appears to pose the greatest risk when it infects pregnant women, given its ability to cause microcephaly in babies, a condition defined by small head size that can lead to developmental problems. The current outbreak was first detected in Brazil last year and has since spread rapidly through the Americas.

Florida Surgeon General Celeste Philip said that health officials are not advising pregnant women to move out of the suspected transmission area.

"We do not believe there will be ongoing transmission," Philip said at a press conference in Orlando, citing daily efforts to control the mosquito population in the area.

The local health department is searching for other potential infections, with more than 2,300 people tested so far in the state, is ramping up mosquito control programs and is distributing Zika protection kits to pregnant women at their doctors' offices, Florida officials said.

Federal authorities have already begun to treat the Florida cases as a sign of local transmission. On Wednesday, the US Food and Drug Administration ordered blood banks in Florida's densely populated Miami-Dade County and Broward County to stop collecting blood until they can test each unit or incorporate technologies that can kill blood pathogens.

Residents in the trendy Miami neighborhood thought to harbour Zika said the local spread of the virus had been inevitable, given the large numbers of tourists from other countries with outbreaks.

Damian Jose Delgado, a 35-year-old father of two, said news of Zika's arrival would make him think twice about expanding his family.

"I think I might be done having kids," Delgado said.

EARLY WARNINGS

US health officials have cautioned for months that the summer mosquito season was likely to bring local outbreaks, with Gulf Coast states such as Florida, Texas and Louisiana on the frontlines. They expect Zika's spread will be more limited than in Brazil, given widespread US use of screens on windows, air conditioning and mosquito control programs.

"As we anticipated, Zika is now here," Tom Frieden, director of the US Centers for Disease Control and Prevention, said during a conference call on Friday.

"There may be occasional clusters" of mosquito-borne transmission in the continental United States, he said, but little likelihood of widespread transmission.

Frieden said the agency was not recommending that people, including pregnant women, limit travel to Florida, including the neighborhood where people may have been infected. He said CDC would reassess that advice if there is evidence of Zika spreading rapidly in the area. - Reuters