"We were surprised, given the numbers actively being investigated," said CDC Deputy Director Anne Schuchat. "We were concerned enough that we thought it was important to share that information. ... We are seeing more than we expected to see."

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Even if all the cases are not ultimately confirmed, Schuchat said, the growing number of reports suggests that sexual transmission of the virus is more possible than researchers previously had realized. That development could further complicate efforts to stop the spread of Zika, as well as force couples to contemplate abstaining from sex or using condoms to prevent transmission, particularly when a woman is pregnant.

Officials said the same scenario played out in most, if not all, of the suspected cases: A man who had traveled to a Zika-affected area returned home to the United States and had sex with a female partner, who soon began to display symptoms consistent with the virus. The agency did not identify the states where these cases are being pursued.

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"Like previously reported cases of sexual transmission, these cases involve possible transmission of the virus from men to their sex partners," the CDC wrote in announcing the new cases. "At this time, there is no evidence that women can transmit Zika virus to their sex partners; however, more research is needed to understand this issue."

Schuchat said that in each of the current cases under investigation, the man had displayed symptoms of Zika, such as fever, rash or joint pain -- a notable detail given that as many as 80 percent of people infected never experience symptoms. She said researchers still aren't sure whether only men who have symptoms can transmit the virus to their partners, but it's a possibility scientists are exploring.

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In addition, she said that because the Zika virus is not yet being widely transmitted by mosquitoes in the United States, as in other affected countries, that should make it easier to identify sexually transmitted cases here.

The issue is among the many questions that remain unanswered about Zika, a once-obscure virus that has spread explosively throughout the Americas in recent months and is suspected of being linked to serious birth defects and a rare autoimmune disease. Scientists do know that the vast majority of Zika infections result from mosquito bites, but proof that the virus can be transmitted through sex continues to grow.

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"We have increasing evidence that this is a route of infection, but we don't know yet how much of an absolute risk it is," Schuchat said. Researchers know that the virus can linger in semen, for instance, but they aren't sure how long it might be there to potentially infect someone else.

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Earlier this month, Dallas health officials reported that a local resident had been infected with the Zika virus by having sex with a person who had contracted the disease while traveling in Venezuela. The Dallas case marked the first local transmission of Zika in the United States -- at least during the current outbreak.

Years earlier, in 2008, a Colorado researcher named Brian Foy came to the conclusion that he had acquired Zika while traveling in Senegal and had later sexually transmitted the disease to his wife, who had not left Colorado. He wrote about the experience in an academic article in 2011.

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"It was clear that she got Zika and I had Zika, and so we made the connection that I certainly did transfer it to her," Foy told The Washington Post recently.

Both the CDC and the World Health Organization have issued guidelines urging the use of condoms to prevent the spread of the virus, particularly when a woman is pregnant or might become pregnant. Officials also have recommended couples consider abstaining from sex for the duration of a pregnancy if the male partner has recently visited a Zika-affected country.

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The CDC on Tuesday reiterated those recommendations, saying that "these new reports suggest sexual transmission may be a more likely means of transmission for Zika virus than previously considered." The agency also has advised women who are pregnant or plan to become so to avoid traveling to Zika-affected areas.

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Also Tuesday, the CDC added two more countries -- Trinidad and Tobago, as well as the Marshall Islands -- to a rapidly expanding list of places on its Zika travel warning. The agency continues to encourage anyone traveling to areas with an ongoing Zika outbreaks to take precautions against mosquito bites, such as wearing long sleeves and using repellent.

On Wednesday, a Senate health committee will hold a hearing with Schuchat and other federal health officials about what the government is doing to prevent the spread of Zika abroad and limit infections in the United States. President Obama this week asked Congress to set aside $1.9 billion to respond to the massive outbreak and its complications, which the WHO has deemed a public health emergency of global concern. Obama said significant resources are needed to help ramp up surveillance efforts, control mosquito populations and accelerate research into new vaccines and diagnostic tests.

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This post has been updated.