The World Health Organization (WHO) voiced concern on Wednesday over a report that the Zika virus had been sexually transmitted in the United States and called for further investigation into the mosquito-borne virus linked to birth defects.

The first known case of Zika virus transmission in the United States was reported in Dallas, Texas on Tuesday by local health officials, who said it likely was contracted through sex and not a mosquito bite.

"We certainly understand the concern. This needs to be further investigated to understand the conditions and how often or likely sexual transmission is, and whether or not other body fluids are implicated," WHO spokesman Gregory Hartl told Reuters.

"This is the only the second mooted case of sexual transmission," he said, referring to media reports about a case of an American man who returned from Senegal and is suspected of infecting his wife.

The virus, linked to babies born with abnormally small heads and birth defects in Brazil, is spreading rapidly in the Americas and the WHO declared an international public health emergency on Monday about the condition known as microcephaly.

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The WHO global response team will discuss the sexual transmission report among other issues at its daily meeting later on Wednesday, Hartl said.

"There are many things we don't know about Zika," he said. "Lots of surveillance is needed...We have our team set up and are sure there will be lots of progress quickly."

For now the key in infected areas is to try to control mosquitos and for people to wear adequate clothing, use insect repellent and sleep under bednets, Hartl said.