Men who have been diagnosed with the Zika virus or have symptoms of the illness should wait at least six months before having unprotected sex, US health authorities said.

Meanwhile, women confirmed to have Zika or who have its symptoms should wait at least eight weeks after the illness first appears before trying to become pregnant, the US Centers for Disease Control and Prevention said in its updated interim guidelines on ensuring healthy pregnancies and babies amid the outbreak.

The recommended waiting periods are the same for sexually active couples who are not trying to get pregnant.

Zika has been linked to microcephaly in babies born to mothers infected while pregnant.

Microcephaly is a serious birth defect that leaves babies with small heads and incomplete brain development.

There is no vaccine to prevent Zika virus and no medicine to treat it.

Although Zika is generally spread by mosquitoes, it can also be sexually transmitted. The CDC has reported six cases of sexually transmitted Zika in the US since February, all by men infected while traveling in Latin America who passed it on to their female partners.

Brazil is the epicenter of the current outbreak, with about 1.5 million people infected, although most cas only cause mild symptoms including fever, rash, pink eye, headache and muscle and joint pain.

The CDC guidance was based on the longest-known risk period, multiplied by three, the health authority said in a news release.

In one case, Zika was detected in an infected man's semen 62 days after the onset of symptoms, the CDC said.

Men with pregnant partners are advised to use condoms for vaginal, anal or oral sex or to abstain from sex throughout the duration of the pregnancy.

For men and women without symptoms who may have been exposed to Zika through travel or sexual contact, the CDC recommends waiting at least eight weeks before trying to become pregnant.

The latest CDC figures show 273 cases of Zika in the United States, all related to travel to areas where the virus is actively transmitted.

Among US overseas territories, Puerto Rico is the most affected, with 261 cases, mostly transmitted by mosquitoes.

The CDC said increasing access to contraception is critical to fighting Zika in Puerto Rico, where two-thirds of pregnancies are unintended. The agency estimated 138,000 women there may be at risk of unintended pregnancy.