Aedes aegypti mosquitoes are seen in a lab of the dengue branch of the U.S. Centers for Disease Control and Prevention in San Juan, Puerto Rico, March 6, 2016. Alvin Baez/Reuters

U.S. health officials are recommending that women wait at least two months, and men at least six, before attempting to conceive after infection with Zika, a virus linked to thousands of suspected cases of birth defects in Brazil.

The new guidance, issued on Friday by the Centers for Disease Control and Prevention, follows prior recommendations by the agency that focused on preventing infections in women who were already pregnant.

Health officials also said they plan to expand the availability of more effective contraceptives in Puerto Rico, the first U.S. territory to report a Zika outbreak, with 261 cases reported to date and thousands more expected in the coming months. The virus was first detected in Brazil last year and has spread rapidly through Latin America and the Caribbean.

"Mounting evidence supports a link between Zika and microcephaly, a birth defect that is a sign of incomplete brain development and possibly other problems, such as miscarriage," Dr. Denise Jamieson, a CDC expert on pregnancy and birth defects, told reporters during a conference call on Friday.

For men and women with possible exposure to Zika, the CDC recommends they wait at least two months before attempting conception.

The guidelines are CDC's best guess based on limited available scientific evidence on how long Zika persists in blood or semen.

"Unfortunately, there is still a lot that we don't know," said Jamieson, adding that the recommendations will continue to evolve as more is learned about the virus.

For people living in areas with a Zika outbreak, the CDC suggested couples delay pregnancy entirely, and urged doctors to counsel patients about the risks of exposure during pregnancy.

To prevent transmission in women who are already pregnant, the CDC reiterated advice that men who live in or have visited areas with Zika transmission should use condoms every time they have sex or consider abstaining for the duration of the pregnancy.

CDC officials are especially worried about women living in Puerto Rico, where Zika is expected to infect hundreds of pregnant women.

In a paper released on Friday, the CDC said two-thirds of pregnancies in Puerto Rico are unintended, compared with roughly 50 percent in the general U.S. population. The researchers estimate as many as 138,000 Puerto Rican women of reproductive age are at risk for unintended pregnancy.

Many of these women are not using highly effective forms of long-acting contraception such as intrauterine devices (IUDs) and hormonal implants, they said.

To address these issues, the U.S. Department of Health and Human Services is coordinating with federal, local and private partners to provide increased access to effective contraceptives in Puerto Rico.

The Centers for Medicare and Medicaid Services will offer additional guidance to U.S. states and territories on how their Medicaid programs can support Zika response, including coverage for contraception.

Family planning services are a mandatory benefit under Medicaid, Jamieson said.

Zika has not been proven to cause microcephaly in babies, but there is growing evidence that suggests a link. The condition is defined by unusually small heads that can result in developmental problems.

Brazil said it has confirmed more than 900 cases of microcephaly and considers most of them to be related to Zika infections in the mothers. Brazil is investigating nearly 4,300 additional suspected cases of microcephaly.