Federal health authorities on Monday urged pregnant women not to visit a South Florida neighborhood where new cases of the Zika virus have emerged, the first time officials have warned against travel to part of the continental United States because of the outbreak of an infectious disease.

Officials issued the “unprecedented” warning following the identification of 10 new infections in a dense urban pocket north of downtown Miami and after “aggressive” efforts to combat Zika’s mosquito-borne spread had proved insufficient.

“It is truly a scary situation,” Tom Frieden, director of the Centers for Disease Control and Prevention, said. “This is a really tough mosquito to control.”

The travel warnings and growing outbreak mark a troubling but not unexpected turn for efforts to stem the virus’s spread through the United States, and they could have profound impacts on Florida’s tourism-heavy economy. It also demonstrates how even the best prepared communities may struggle to deal with a virus that spreads so readily.

Most people infected with Zika don’t show symptoms, but the virus can have devastating consequences during pregnancy. A woman infected with Zika can pass the disease to her fetus, stunting brain development and causing other severe defects.

Nearly all of the 1,661 Zika infections in the continental U.S. came from travelers to Puerto Rico or nearly 50 countries such as Brazil and Colombia, where outbreaks are widespread. But 14 cases of Zika infections caused by mosquito bites in the United States have been announced by health officials since Friday, leading to worries of an expanding local spread.

The primary species of mosquitoes that has so far carried the virus is present in 30 U.S. states, and the South’s warm, humid climates are particularly fertile ground for Aedes aegypti, a breed that only bites humans and thrives in urban neighborhoods. A second mosquito species, Aedes albopictus, can also carry the virus.

The CDC’s travel warning covered only about a square mile of north Miami, Frieden said, because that mosquito typically travels only 150 meters during its lifetime. But the threat could arise in other communities across the United States, because 40 million people travel to Zika-affected areas every year and could silently pass on the virus to mosquitoes and lead to new outbreaks.

Experts have been expecting local transmission of Zika. Miami “is just the one outbreak we know about,” Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine, said. “I think it’s equally possible that multiple outbreaks are simultaneously occurring up and down the Gulf Coast and Florida.”

The CDC is advising that all pregnant women should be asked about travel to Zika-infested areas during routine prenatal visits. Any pregnant women who have traveled to Zika areas, including this area of Florida, are advised to get tested for Zika.

Women who have visited the affected area north of Miami since June 15, when Zika’s local spread appears to have begun, were told to avoid getting pregnant for at least eight weeks.

Women were also advised to use protection during sex, because the virus can be transmitted sexually. Men were told to wait at least six months before attempting conception if they see symptoms of the Zika virus disease, including fever, rash and joint pain.

There is no medical treatment for Zika, and no vaccine.

The Zika-affected area of Miami, bounded by Biscayne Boulevard and Interstate 95, encompasses a dense, diverse mix of urban land ranging from high-end art galleries to aging bungalows, making it more difficult for crews to control for mosquitoes and eradicate standing water, the CDC said.

The area also includes Wynwood, a once-predominantly Puerto Rican enclave known as Little San Juan that has become one of Miami’s trendiest neighborhoods. A magnet in the ’80s for artists seeking cheap studio space, Wynwood is now renowned for its outdoor art, and its blocks of intricate murals and warehouse graffiti now draw visitors from around the world.

The CDC has dispatched an emergency-response team of experts in birth defects, mosquito control and other fields to assist in combatting the outbreak. Six of the 10 new cases identified Monday were people who showed no symptoms but were identified through the local health department’s door-to-door survey.

More alarming, Frieden said, is that “aggressive” strategies aimed at slowing Zika’s spread “don’t seem to be working as well as we would have liked,” possibly because mosquitoes have become resistant to insecticides or because they are thriving in “cryptic breeding places,” small puddles of standing water where eggs can continue to hatch.

Frieden and other officials say they expect additional cases of “homegrown Zika” within the coming weeks.

Zika’s spread in Florida has cast a new spotlight on congressional inaction that prevented further funding to help fight the virus. Republicans and Democrats in Congress have been deadlocked for months over the fate of a $1.1 billion spending bill that would help fight the spread of Zika. Negotiations over a bipartisan spending package crumbled in late June, with neither side willing to reopen talks. Congress is out of session for August, and lawmakers are not expected to resume legislative business until after Labor Day.

Senate Democrats have blocked the funding package drafted by congressional Republicans over politically motivated language, including provisions that would deny Zika-related funds from being sent to Planned Parenthood and loosen environmental regulations on pesticides. Senate Minority Leader Harry Reid, D-Nevada, last week urged Senate Majority Leader Mitch McConnell, R-Kentucky, to call a special session of Congress, saying “Zika is a public health emergency that requires immediate bipartisan action, and the American people can’t afford to wait several weeks for a response.”

Senate Republicans responded by blaming Democrats for blocking the House-passed spending bill. McConnell spokesman Don Stewart said, “The White House has acknowledged that there are hundreds of millions in unspent Zika funds available already, but have not explained why they’ve been so slow in using the funding they already have.”

Sen. Marco Rubio, R-Florida, last week issued a statement urging President Obama to immediately tap the remaining Ebola funds, but on Monday he also pushed for Congress to come back to Washington early to vote on a long-term Zika spending bill.

“A week ago, before the cases were announced, I had asked President Barack Obama to take $300 million that’s disposable, that he has under his control,” Rubio said at an event in Clearwater, Florida. “I’m prepared to go back in a moment’s notice and vote on this and get it done quickly given the state of affairs now.”

The travel warnings and Zika’s spread have forced Florida to confront a unique threat to its tourism industry, the bedrock of the state’s economy. Businesses across the state rely heavily on flocks of travelers to its beaches, theme parks and tourist traps, and Florida last year became the first state to welcome more than 100 million tourists in a single year.

More than a million Floridians work directly in the state’s tourism industry, and tourism and travel spending in the state climbed to $89 billion last year, state economic data show. Those dollars filter down into virtually every industry, including hotels, restaurants, health care and housing.

The Zika warnings, coming in the thick of mosquito season, remain clustered in a square-mile pocket 220 miles south of Orlando’s theme-park mecca. But businesses say they’re worried that even the appearance of a dangerous outbreak could shake confidence among young families, the backbone of the state’s tourism economy.

“That’s absolutely what’s frightening a lot of businesses here: Tourism is Florida’s major economy, and the service sector is hugely dependent on people coming from everywhere,” Susan MacManus, a political-science professor at the University of South Florida in Tampa, said. “Businesses want to believe it’s very localized in Miami. But on the other hand, they’re realistic — and fearful it’ll go beyond that.”

Some countries have issued warnings about travel to Florida, including the United Kingdom and Ireland, which over the weekend advised women who are pregnant or trying to become pregnant to “consider postponing their travel” to the state.

Florida Gov. Rick Scott, a Republican, on Monday welcomed tourists to come to the Sunshine State. “While I encourage all residents and visitors to continue to use precaution by draining standing water and wearing bug spray, Florida remains safe and open for business,” Scott said.

Some potential tourists, however, are already having second thoughts. Mike Rigelsky, a lawyer in Ohio, last fall planned the Disney World vacation of a lifetime: A week of theme parks in the Orlando sun, including a planned family photo shoot during which his sister would announce her pregnancy.

Yet as concerns mounted over Zika, Rigelsky and his sister did what scores of bridesmaids, parents and other vacationers did before them: They canceled, unwilling to risk an infection.

“A lot of people were hitting the panic button, but we were very clinical about it: We took it from the perspective that we just didn’t know enough,” Rigelsky said. We “had to make a decision: Do you take a risk, however minimal, just for the sake of a vacation?”

Writers Joel Achenbach and Kelsey Snell contributed to this report.