Women infected with the mosquito-borne Zika virus should continue to breastfeed their babies as there is currently no proof of a risk of transmission, the World Health Organization said on Thursday.

"In light of available evidence, the benefits of breastfeeding for the infant and mother outweigh any potential risk of Zika virus transmission through breast milk." the WHO said in interim recommendations to authorities in countries affected by the outbreak.

The WHO noted that the Zika virus had been detected in the breast milk of two infected mothers, but added "there are currently no documented reports of Zika virus being transmitted to infants through breastfeeding."

"A systematic review of evidence will be conducted in March 2016 to revise and update these recommendations," it added.

Cases of active Zika transmission have been reported in 28 countries and territories in the Americas and Caribbean, with 1.5mn in Brazil, the hardest-hit country.

In nearly all Zika cases, symptoms are mild, resembling those of flu. However, the growing belief that Zika can also trigger microcephaly in babies born to mothers infected while pregnant has spread international alarm.

Microcephaly is a congenital condition that causes abnormally small heads and hampers brain development.

There is currently no cure or vaccine against the Zika virus.

On Wednesday, WHO chief Margaret Chen warned that the virus will be hard to stamp out.

The situation "could get worse before it gets better," she said in Rio de Janeiro after a fact-finding mission to Brazil.

"We are dealing with a tricky virus, full of uncertainties, so we should be prepared for surprises," she said.

Chan added that up to 46 countries have reported some level of evidence of Zika infection and that 130 countries are home to the Aedis aegypti mosquito that carries the virus, meaning the eventual spread of the disease could be enormous.