Mosquitoes are transmitting the Zika virus in the popular tourist spot Miami Beach, Florida, and pregnant woman should think twice before traveling there, a US federal health agency warned.

“Pregnant women and their sexual partners who are concerned about potential Zika virus exposure may also consider postponing nonessential travel to all parts of Miami-Dade County,” cautioned the Centers for Disease Control and Prevention in a statement.

This comes a few weeks after the first known US-based mosquito transmission of Zika occurred in the Miami neighborhood of Lynwood.

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The Zika virus is considered a global health emergency and is a major health concern, particularly for pregnant women, in Latin America and the Caribbean. This is the first expansion of the virus, which can also be transmitted sexually, in the US.

While Zika has existed for decades, the first recent outbreak occurred in February 2015 in northeast Brazil. 

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Often people who contract Zika don’t experience symptoms, but mothers with Zika often give birth to babies with microcephaly, where a baby is born with a smaller-than-average head.

There is no known treatment for the virus, and there isn’t yet a vaccine. Efforts to create one have just recently started.

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While more than 1,650 people in the US have been infected with Zika, most have occurred while traveling abroad. By January 2016, 1.5 million people had been infected.

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.