MIAMI — The mosquitoes spreading Zika in Miami are proving harder to eradicate than expected, the nation’s top disease-fighter said Tuesday as authorities sprayed clouds of insecticide in the ground zero neighborhood, emptied kiddie pools and handed out cans of insect repellent to the homeless.

Dr. Tom Frieden, director of the Centers for Disease Control and Prevention, said the mosquito-control efforts in the bustling urban neighborhood aren’t achieving the hoped-for results, suggesting the pests are resistant to the insecticides or are still finding standing water in which to breed.

“We’re not seeing the number of mosquitoes come down as rapidly as we would have liked,” he said in an interview with The Associated Press.

Mosquito control experts said that’s no surprise to them, describing the Aedes aegypti mosquito as a “little ninja” capable of hiding in tiny crevices, sneaking up on people’s ankles, and breeding in just a bottle cap of standing water.

Fifteen people have become infected with Zika in Miami’s Wynwood arts district, officials said Tuesday. These are believed to be the first mosquito-transmitted cases in the mainland U.S., which has been girding for months against the epidemic coursing through Latin America and the Caribbean.

On Monday, the CDC instructed pregnant women to avoid the neighborhood, marking what is believed to be the first time in the agency’s 70-year history that it warned people not to travel somewhere in the U.S. The Zika virus can cause severe brain-related defects, including disastrously small heads.

At the same time, U.S. health authorities have said they don’t expect major outbreaks in this country, in part because of better sanitation and the use of air conditioners and window screens.