Public health officials in El Salvador have advised women to delay pregnancy for the next two years to prevent children from developing birth defects from the mosquito-borne Zika virus.

Eduardo Espinoza, the country's vice-minister of public health, said on Thursday that women who were already pregnant should stay covered outdoors to reduce the risk of mosquito bites.

"We'd like to suggest to all the women of fertile age that they take steps to plan their pregnancies, and avoid getting pregnant between this year and next," he said.

The government decided to make the announcement after 5,397 cases of the Zika virus were detected in El Salvador in 2015 and the first few days of this year, Espinoza said.

READ MORE: Zika virus cases spread in Latin America, Caribbean

Official figures show 96 pregnant women were suspected of having contracted the virus, but so far none has had babies born with microcephaly, or abnormally small heads, which the disease can cause.

In Colombia, which has the second-highest Zika infection rate after Brazil, the government is also advising women to delay becoming pregnant, but only for six to eight months.

Vaccine hopes

President Dilma Rousseff said on Thursday that Brazilian researchers were working with domestic and foreign laboratories to try to develop a vaccine for the Zika and dengue viruses.

READ MORE: World's first malaria vaccination approved

The Zika virus is transmitted by Aedes species mosquitoes, which also spread dengue and chikungunya viruses. Zika causes a mild illness with fever, rash, joint pain and conjunctivitis, with symptoms usually lasting under a week.

But in pregnant women, the virus can spread to the foetus and cause microcephaly - a condition involving brain shrinkage that severely limits a child's intellectual and physical development - or death.

Brazil has been the hardest hit, documenting more than 3,500 cases of microcephaly between October 2015 and January 2016

Health experts are unsure why the virus, which was first detected in Africa in 1947 but unknown in the Americas until last year, is spreading so rapidly in Brazil and neighbouring countries.

Last week, the United States warned pregnant women to avoid travelling to 14 countries and territories in the Caribbean and Latin America owing to the virus.

The level two travel alert applies to Brazil, Colombia, El Salvador, French Guiana, Guatemala, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay, Suriname, Venezuela and Puerto Rico.