GENEVA (Reuters) - The World Health Organization on Wednesday advised women in areas with the Zika virus, especially during pregnancy, to cover up against mosquitoes and practice safe sex but also reassured them that most would give birth to "normal infants".

Acknowledging "understandable concerns" due to the risk of babies with birth defects, the WHO issued recommendations but conceded that "many unknowns" still surround the virus sweeping across the Americas.

The WHO declared an international health emergency 10 days ago after a spike in cases of newborns with microcephaly in Brazil linked to the mosquito-borne Zika virus, which has now been found in more than 30 countries.

On Wednesday, the United Nations agency did not recommend travel restrictions, instead suggesting that women "determine the level of risk they wish to take" and consult their doctors or authorities if traveling and on return.

"Whether and when to become pregnant should be a personal decision, on the basis of full information and access to affordable, quality health services," it said.

Women who are pregnant or planning to become pregnant should protect themselves against bites of the Aedes mosquito that transmits Zika, the Geneva-based U.N. agency said.

It advised using insect repellent with DEET; wearing light-colored clothes that cover the body; closing doors and windows; sleeping under mosquito nets; eliminating potential mosquito breeding sites in containers that can hold even small amounts of water, such as buckets, flower pots and tyres.

More evidence is needed to confirm whether sex commonly transmits the Zika virus, it said, noting that Zika has been found in semen and citing a report of sexual transmission in the United States. There is no vaccine or treatment for Zika.

"Until more is known, all men and women living in or returning from an area where Zika is present - especially pregnant women and their partners - should be counseled on the potential risks of sexual transmission and ensure safe sexual practices. These include the correct and consistent use of condoms, one of the most effective methods of protection against all sexually-transmitted infections," the WHO said.

Pregnant women in general, including those who develop symptoms of Zika virus infection, should be closely monitored.

But the WHO was also somewhat reassuring, declaring: "Most women in Zika-affected areas will give birth to normal infants."

Microcephaly is a rare condition where a baby is born with a small head. It can result in developmental delays as well as seizures, hearing loss, vision problems and trouble swallowing.

"Early ultrasound does not reliably predict microcephaly except in extreme cases," the WHO said.

Zika virus has been detected in breast milk but there is currently no evidence that the virus is transmitted to babies through breastfeeding, it said, reiterating its recommendation of exclusive breastfeeding for the first 6 months of life.

"Women who wish to terminate a pregnancy due to a fear of microcephaly should have access to safe abortion services to the full extent of the law," the WHO said.

(Reporting by Stephanie Nebehay; Editing by Mark Heinrich)