The mosquitoes spreading Zika in Miami are proving more difficult to eradicate than expected, the nation's top disease-fighter said as authorities sprayed the ground-zero neighbourhood, emptied paddling pools and handed out cans of insect repellent to the homeless.

Tom Frieden, director of the Centres for Disease Control and Prevention, said the mosquito-control efforts were not achieving the hoped-for results, suggesting the pests were resistant to insecticides and were still finding standing water in which to breed in the bustling urban neighborhood.

"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 was 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.

Fourteen people are believed to have become infected with Zika from bites in Miami's Wynwood arts district — the first mosquito-transmitted cases on record in the mainland US, 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 neighbourhood, 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 US. The Zika virus can cause severe brain-related defects, including disastrously small heads.

The Zika virus - in pictures Show all 5 1 /5 The Zika virus - in pictures The Zika virus - in pictures A three-month-old, who has microcephaly, in Recife, Brazil. A rise in microcephaly cases is thought to have been caused by the spread of the Zika virus in affected countries Getty Images The Zika virus - in pictures A mother holds her baby who has microcephaly Getty Images The Zika virus - in pictures A five-month-old baby, who has microcephaly, in Recife, Brazil Getty Images The Zika virus - in pictures A pediatric infectologist examines a two-month-old baby, who has microcephaly, in Recife, Brazil Getty Images The Zika virus - in pictures A baby affected with microcephaly

At the same time, American 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.

Miami-Dade County mosquito control inspectors went door to door in Wynwood on Tuesday, handing out information, checking tyres and other objects for standing water, and dipping cups to take water samples from vacant lots, building sites and backyards. In one lush yard, an inspector tipped over a paddling pool and a cooler full of water.

Team GB talk Zika

Daily aerial spraying for adult mosquitoes and larvae has been approved for the next four weeks over a 10-square-mile area around Wynwood, county officials said. Also, the city of Miami is running extra street-sweeping routes to remove the litter and stagnant water that can serve as breeding grounds.

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Because of environmental regulations governing which chemicals can be used as insecticides, mosquito control authorities cannot easily switch to another compound if bugs prove resistant to it.

Nothing has worked to stop this mosquito elsewhere in the world except for the introduction of mosquitoes modified to pass on genes that kill their offspring, said Michael Doyle, executive director of the Florida Keys Mosquito Control District. And the Food and Drug Administration has not given approval to that approach in the US.