Katharine Lackey, and Doug Stanglin

USA TODAY

Four people infected with Zika in South Florida likely contracted the virus from local mosquitoes, marking the first time the disease has been transmitted by the insects in the continental U.S., Florida Gov. Rick Scott said Friday.

Florida's Department of Health believes the mosquitos that transmitted the virus are active in a small area just north of downtown Miami, Scott said at a news conference in Orlando. No mosquitoes in the state have tested positive for Zika, he added.

The four Zika patients work in the same area, although not at the same company, and are not related, said Thomas Frieden, director of the Centers for Disease Control and Prevention.

“All the evidence we have seen indicates that this is mosquito-borne transmission that occurred several weeks ago in several blocks in Miami,” he said.

More than 1,650 people in the U.S. have contracted Zika, but until now all were linked to travel to areas outside the U.S. with outbreaks of the virus. Zika, which can cause devastating birth defects, is spreading rapidly in more than 30 countries and territories in the Caribbean and Latin America.

Zika Q&A: What you need to know about the virus

The virus primarily spreads through bites from infected mosquitoes, but can also be spread through sex. Only one in five people with Zika develop symptoms, which include rash, fever, muscle aches and headaches.

The four cases in South Florida involved one woman and three men in Miami-Dade and Broward counties, Scott said.

"If you live in this area and want to be tested, I urge you to contact the county health department, which stands ready to assist you," he said.

Florida Commissioner of Agriculture and Consumer Services Adam H. Putnam called on residents to eliminate any standing water around their homes, get rid of old tires in their yards that could hold water and make good use of insect repellant.

He said the situation in Florida is far different than in many Latin American countries, which may not have widespread use of air conditioning and window screens.

Florida Surgeon General Celeste Philip said officials don't believe expect ongoing transmission of the virus, but they will monitor the situation.

“As with most emerging health threats, we learn more about Zika each day, but we recognize that the unknown can be scary, especially for pregnant women," she said. "We’re committed to sharing as much as we can as soon as we can. Our top priority is the safety and well-being of all people in Florida."

Frieden said the CDC would continue to support Florida's efforts to investigate and respond to Zika cases and will reassess the situation and its recommendations daily.

The Food and Drug Administration on Thursday asked all blood centers in Miami-Dade, Broward and surrounding areas to stop taking donations until they can be tested for the virus.

Blood collection centers across the country also shouldn't accept any donations from people who have traveled to Miami-Dade and Broward counties within the past four weeks, the FDA added.

OneBlood, Florida's main blood supplier, said in a statement it would work as quickly as possible to comply with the FDA's "unanticipated" request. The company said it would start testing all its collections Friday.