The World Health Organization has changed its advice to travellers returning from areas where the Zika virus is actively spreading, saying both men and women should now practise safe sex or abstinence for six months.

The new guidance is a change from the health agency's interim recommendation on June 7, which referred only to men and had a shorter timeframe of at least eight weeks.

The WHO said on Tuesday that the updated guidance applied to all travellers, whether or not they show symptoms of the virus.

The update was based on new evidence on Zika transmission from asymptomatic males to their female partners and a symptomatic female to her male partner, as well as evidence that Zika is present in semen for longer than thought, the WHO said.

Many people infected with Zika will not have symptoms, or will only have mild ones - a fever, rash and muscle or joint aches.

Zika infections in pregnant women have been shown to cause microcephaly - a severe birth defect in which the head and brain are undersized - as well as other brain abnormalities.

The connection between Zika and microcephaly first came to light last autumn in Brazil, which has since confirmed more than 1,800 cases of microcephaly.

In adults, Zika infections have also been linked to a rare neurological syndrome known as Guillain-Barre, as well as other neurological disorders.

Sexual transmission of Zika had been reported in 11 countries by August, mainly through vaginal intercourse. There was a first documented case of a man catching the virus through anal sex in February 2016 and a suspicion of Zika transmission through oral sex in April.

On Wednesday, Malaysia confirmed its first case of the Zika virus in a pregnant 27-year-old woman.

INFOGRAPHIC: What is the Zika virus?

Last week, the country had confirmed the first imported case of Zika in a 58-year-old woman who had visited Singapore which itself stands at a risk of having patients infected with Zika and dengue fever.

As of Monday, the number of dengue fever cases in Singapore at 11,343 had surpassed the number of cases in the whole of 2015, at 11,286.

The new guidance came after doctors discovered the virus in the sperm of an Italian man 188 days after he first showed Zika symptoms.

In another Zika sufferer, the concentration of the virus in his semen was 100,000 more than that in his blood 14 days after he was diagnosed.

Evidence on persistence of the virus in semen and its infectiousness and impact on sexual transmission remains limited and the guidance will be updated again when there is more information, WHO said.

The health body advises that pregnant women should not travel to areas with ongoing Zika virus transmission, and has warned people travelling to the Paralympic Games, which starts on Wednesday in Rio de Janeiro, to take precautions against mosquito bites.

"We think that the risk for travellers and athletes is low, but it's not zero," WHO spokesman Tarik Jasarevic said.