ORLANDO, Fla. -- Florida is believed to have the first cases of Zika virus transmitted by mosquitoes on the U.S. mainland, health officials said Friday.

One woman and three men in Miami-Dade and Broward counties likely contracted the virus through mosquito bites, Gov. Rick Scott said during a news conference in Orlando. Mosquitoes in the area are being collected and tested, but so far none have tested positive for Zika.

In a news briefing this afternoon, CDC director Dr. Tom Frieden said that although there is no way to retrospectively confirm without a doubt that the virus was locally transmitted by mosquitoes, "all the evidence we've seen is that this is mosquito-borne."

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"As we have anticipated, Zika is now here," Frieden said.

He added there may be additional cases in the area that haven't been reported, because most people infected with the virus don't experience any symptoms.

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.

"This is not just a Florida issue. It's a national issue - we just happen to be at the forefront," Scott said.

Health officials believe the infections occurred in a small area just north of downtown Miami, in the popular Wynwood arts district, Scott said.

It's the only part of the state currently being tested for potential local transmissions of Zika, Scott said. The transmission is believed to have occurred in early July.

Women in the area who are pregnant or thinking about becoming pregnant are urged to contact their doctors and the county health department for Zika prevention kits.

Federal health officials have not recommended that pregnant women avoid travel to South Florida.

"There are a series of factors we'll have to look at. The number of cases, the relationship in geography of those cases, how closely linked they are in time, as well as a series of other factors that we will use to determine what recommendations we issue in terms of travel guidance," U.S. Surgeon General Dr. Vivek Murthy said Thursday.

Zika primarily spreads through bites from tropical mosquitoes, but it also can be spread through sex. In most people, the virus causes only mild illness, but infection during pregnancy can lead to severe brain-related birth defects for the fetus. There is no vaccine.

This week, a pharmaceutical company administered its first dose of an experimental Zika vaccine to a volunteer, CBS News correspondent David Begnaud reports.

Since February, over 380 Zika cases related to travel have been confirmed in Florida, including 151 cases in Miami-Dade and Broward counties.

Mosquito control

The Aedes aegypti mosquito that spreads Zika and other viruses is found in the southern U.S. While health officials have predicted that mosquitoes in the continental U.S. would begin spreading Zika this summer, they also have said they expect only isolated clusters of infections and not widespread outbreaks.

Florida should be able to contain the virus to an isolated area because its neighborhoods have better sanitation and living conditions than countries where Zika outbreaks have been widespread, said Adam Putnam, Florida's commissioner for agriculture and consumer services.

The Aedes aegypti mosquito does not travel long distances, so mosquito control efforts are focusing on the immediate areas near where the cases occurred.

Pregnant women in South Florida and all places where the Aedes aegypti mosquito is present should take steps to prevent mosquito bites, Frieden emphasized. These include using an insect repellent with DEET, wearing long pants and long-sleeve shirts, staying indoors with air conditioning, and eliminating all standing water.

Protecting the blood supply

The U.S. Food and Drug Administration has told blood centers in Miami-Dade and Broward counties to suspend collections until they can screen each unit of blood for the Zika virus with authorized tests. Neighboring counties have been urged to implement the same precautions, and visitors to South Florida in the last month are encouraged to defer donations as well.

The FDA previously advised U.S. blood banks to refuse donations from people who recently traveled to areas outside the country that have Zika outbreaks.

Florida's main supplier of blood, OneBlood, said it was working as quickly as possible to comply with the FDA's recommendation and would start testing all its collections for Zika on Friday.