WASHINGTON — The director of the Centers for Disease Control and Prevention says officials are finding it challenging to eradicate mosquitoes in a part of Miami where 14 people appear to have contracted the Zika virus.

CDC Director Tom Frieden told ABC's "Good Morning America" Tuesday that officials issued a rare travel warning advising pregnant women to avoid Miami's Wynwood arts district because mosquito counts are still high in the area.

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Frieden said it's possible mosquitoes are resistant to the insecticide being used, but it could take weeks for federal and state officials to figure that out. He said there also could be breeding sites that haven't been destroyed.

He said mosquito control is difficult in the neighborhood because it has industrial, commercial and residential development. The mosquito that carries the virus is generally difficult to eradicate.