Florida's governor said on Friday the state has concluded that four mysterious Zika infections likely came from mosquitoes in the Miami area.

Gov. Rick Scott said Friday that no mosquitoes in the state have tested positive for Zika. But he says one woman and three men in Miami-Dade and Broward counties likely contracted the virus through mosquito bites.

More than 1,650 Zika infections have been reported in the U.S., but the four patients in Florida would be the first not linked to travel outside the U.S. mainland.

Scott says health officials believe the infections occurred in a small area just north of downtown Miami.

Zika primarily spreads through bites from tropical mosquitoes. In most people, the virus causes only mild illness, but infection during pregnancy can lead to severe brain-related birth defects for the fetus.

Meanwhile, the U.S. Food and Drug Administration has ordered blood banks in Florida's two most densely populated counties to stop collecting blood as health officials determine whether Zika has begun transmission in the continental United States.

Florida has been investigating four possible cases of local transmission in Miami-Dade County and Broward County. It is the first U.S. state to report cases that may not be related to travel to other countries with active outbreaks.

Zika has struck hardest in Brazil, where the outbreak was first detected last year, and has since spread rapidly through the Americas. The virus can cause a rare birth defect, microcephaly, in newborns whose mothers have been infected, and is believed to be linked to Guillain-Barre syndrome in adults.

Zika most commonly infects people via mosquito bite. But reports of the virus being transmitted through sex and blood transfusions has prompted public health officials to recommend additional precautions for sexual partners and blood banks.

In a statement posted online on Wednesday, the FDA said blood centers in the two Florida counties should stop collecting blood until they can test each unit or put in place technology that can kill pathogens in the blood.

The FDA also recommended that nearby counties implement the same measures as it moves to prevent transmission of the virus through the blood supply.

OneBlood, Florida's biggest blood collection center, said it will begin testing all of its collections for Zika virus, effective July 29, using an investigational screening test and that it is working as quickly as possible to comply with the FDA's request.

The FDA has authorized the emergency use of several investigational Zika screening tests, including products made by Hologic Inc and Roche Holding AG.

The agency has also approved a pathogen inactivation technology made by Cerus Corp that kills the virus in blood platelets and plasma. The company is conducting clinical trials to show it can also kill pathogens in red blood cells.

The United States uses roughly 12 million units of red cells, four million plasma units and two million units of platelets a year.