El Salvador has urged women in the Central American nation to delay getting pregnant until 2018, as the Zika virus rages on.

El Salvador, a country where where abortions are illegal and birth control is expensive and hard to attain, has told women to avoid getting pregnant until 2018, to avoid their children developing birth defects from the mosquito-borne Zika virus which has rampaged through the Americas.

The Zika virus is transmitted by the Aedes aegypti mosquito, which is also known to carry the dengue, yellow fever and Chikungunya viruses. 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.

Although research is still underway, significant evidence in Brazil shows a link between Zika infections and rising cases of microcephaly, a neurological disorder in which infants are born with smaller craniums and brains.

"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," said Deputy Health Minister Eduardo Espinoza.

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He said the government decided to make the announcement because 5,397 cases of the Zika virus had been detected in El Salvador in 2015 and the first few days of this year.

Official figures show 96 pregnant women are suspected of having contracted the virus, but so far none have had babies born with microcephaly.

In Colombia, there are 20,297 confirmed cases of the disease, said the national health institute in a epidemiology bulletin – among them 2,116 pregnant women.

The Colombian government is also advising women to put off becoming pregnant, for six to eight months, to avoid potential infection. Officials expect up to 700,000 cases.

There are so far no reported cases of microcephaly or deaths from the virus in Colombia.

Nearly half of Colombia's Zika cases have been reported in the country's Caribbean region, the bulletin said. More than 60 per cent of those infected are women.

Brazil is the country hit hardest by the disease. It has reported around 3,700 cases of microcephaly strongly suspected to be related to Zika.

The World Health Organisation has said as many as 4 million people in the Americas may become infected.