MIAMI - (AP) -- Mosquitoes have apparently begun spreading the Zika virus on the U.S. mainland for the first time, health officials said Friday, a long-feared turn in the epidemic that is sweeping Latin America and the Caribbean.

Four recently infected people in the Miami area -- one woman and three men -- are believed to have contracted the virus locally through mosquito bites, Gov. Rick Scott said.

No mosquitoes in Florida have actually been found to be carrying Zika, despite the testing of 19,000 by the state lab. But other methods of Zika transmission, such as travel to a stricken country or sex with an infected person, have been ruled out.

"Zika is now here," said Dr. Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention.

Still, U.S. health officials said they do not expect widespread outbreaks in this country like those seen in Brazil, in part because of better sanitation, better mosquito control and wider use of window screens and air conditioners.

The virus has triggered alarm across the Western Hemisphere's warmer latitudes. While most people who get Zika don't even know they are sick, infection during pregnancy can cause severe brain-related birth defects, including disastrously small heads.

More than 1,650 people in the mainland U.S. have been infected with Zika in recent months, nearly all while traveling abroad. The four people in Florida are believed to be first to contract the virus from mosquitoes within the 50 states.

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

Florida agricultural officials immediately announced more aggressive mosquito-control efforts, and Florida politicians rushed to assure tourists it's still safe to visit the state.

Some medical experts said pregnant women should not travel to the Miami area, especially if the visit involves spending time outdoors. The CDC is not issuing such advice, however.

Health officials said the U.S. might see small clusters of infections. But "we don't expect widespread transmission in the continental United States," the CDC's Frieden said.

The four Florida infections are thought to have occurred in a small area just north of downtown Miami, in the Wynwood arts district, the governor said.

The area, known for bold murals spray-painted across warehouses, art galleries, restaurants and boutiques, is rapidly gentrifying and has a number of construction sites where standing water can collect and serve as a breeding ground for mosquitoes.

People in Florida's Miami-Dade and Broward counties are being tested to learn whether there are more cases, the governor said.

"If I were a pregnant woman right now, I would go on the assumption that there's mosquito transmission all over the Miami area," warned Dr. Peter Hotez, a tropical medicine expert at the Baylor College of Medicine in Texas.

He said that there are probably more cases that have not been diagnosed, and that people should not be surprised if mosquitoes are soon found to be spreading Zika in Louisiana and Texas as well.

Earlier this week, federal authorities told blood centers in the Miami-Fort Lauderdale areas to stop collecting blood until they screen it for the virus.

Zika's symptoms can include low-grade fever, rash, joint pain, headaches and inflamed eyes and typically last seven to 10 days. None of the four people infected in Florida are showing symptoms anymore, officials said.

Frieden said the evidence suggests that the mosquito-borne transmission occurred several weeks ago over several city blocks.

It's not unusual that no mosquitoes have tested positive for Zika, said C. Roxanne Connelly, a medical entomology specialist at the University of Florida and a past president of the American Mosquito Control Association.

It can take a couple of weeks before an infected person shows symptoms, and by then the mosquitoes that transmitted the virus are dead, she said.

"Believe it or not, it's difficult to find positive mosquitoes even when you're in the middle of an epidemic," Connelly said. "Sometimes you don't know where these people were infected. At home? At work? Where they were playing baseball?"

Zika primarily spreads through bites from a specific species of mosquito that is found in urban parts of the South and peaks in number in August and September. There have been more than 4,700 cases of mosquito-borne Zika in Puerto Rico and other U.S. territories.

The cycle of infection inside a country can start when a blood-sucking mosquito bites a traveler who has returned from abroad with the virus in his or her bloodstream. The mosquito then bites someone else, spreading the virus.

Health officials have been long predicting this would happen in the continental U.S. sometime this summer, probably in Florida and Texas, because of the types of mosquitoes that thrive there and the large numbers of people who travel back and forth to Latin America.

Orange County Mayor Teresa Jacobs, whose jurisdiction includes Walt Disney World and other Orlando-area theme parks, said tourists shouldn't think twice about coming to the Sunshine State. Florida had more than 106 million visitors last year, and tourism is the state's biggest industry.

Florida's theme parks "have mosquito control down like no place else I don't think on the planet," Jacobs said.

Zika-fighting efforts include pesticide spraying, setting of traps and eliminating standing water around homes.

Florida's governor has allocated over $25 million for dealing with Zika, and the White House and the CDC have provided over $10 million. However, Congress left on a seven-week vacation without giving the Obama administration any of the $1.9 billion it sought to battle the virus.

White House spokesman Eric Schultz called that "regrettable" and said: "Today's news should be a wake-up call to Congress to get back to work."

Jenny Gray, who is 27 and works in Wynwood for an art designer, said she will follow the experts' advice to wear insect repellent.

"I don't plan on having kids now, but I do sometime in the future. Better to stay protected," she said. "That really does concern me."

But Phillip Lopez, a 34-year-old Wynwood resident who works at an outdoor bar and exercises outside, said: "It's a concern, but you got to do what you got to do. You can't not go outside."

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Associated Press writers Mike Schneider in Orlando, Florida; Mike Stobbe in New York; and Josh Lederman in Washington contributed to this report.