Scientists say a study involving pregnant women in Brazil “strengthens” the theory that Zika is linked to microcephaly birth defects in babies.
The research confirmed the presence of Zika virus in the amniotic fluid of two women who had had Zika-like symptoms during their pregnancies.
Brazilian experts say this suggests the virus can infect the foetus.
But WHO experts caution the link is not proven and expect to release more information in the next few weeks.
Brazil has seen a rise in microcephaly – babies born with abnormally small heads and, in some cases, problems with brain development – in the last year, at the same time as a rise in the number of people infected with Zika virus.
This has led to a number of studies investigating whether the virus is behind the rise.
The research, published in the journal Lancet Infectious Diseases, involved two women who had fever, rash and muscle aches during their pregnancies.
After ultrasound scans revealed their developing foetuses had microcephaly, scientists ran further amniocentesis checks.
This involved taking a small sample of the amniotic fluid that surrounds the foetus in the womb.
Genetic analysis of this fluid confirmed the presence of Zika virus – discounting similar viruses that may have been responsible.
Lead scientist, Dr Ana de Filippis, from the Oswaldo Cruz Institute in Rio de Janeiro, Brazil, said: “This study reports details of the Zika virus being identified directly in the amniotic fluid of a woman during her pregnancy, suggesting the virus could cross the placental barrier and potentially infect the foetus.”
She added: “This study cannot determine whether the Zika virus identified in these two cases was the cause of microcephaly in the babies.
“Until we understand the biological mechanism linking Zika to microcephaly we cannot be certain that one causes the other, and further research is urgently needed.”
Prof Jimmy Whitworth, at the London School of Hygiene and Tropical Medicine added that while the research cannot prove the link: “This study does strengthen the body of evidence that Zika virus is the cause of foetal microcephaly in Brazil.”
Separately the paper suggests that the virus looks genetically very similar to the Zika virus circulating in French Polynesia in 2013.
But scientists say despite growing research, a lot remains unknown and a number of questions still need urgent answers – including how big the risk of microcephaly is if a woman has Zika virus infection in pregnancy and whether the timing of the infection makes a difference.
Brazil, the country hardest-hit by Zika, has about 508 confirmed cases of microcephaly and is investigating about 3,935 suspected cases.
The ministry said last week that 41 of the confirmed cases of microcephaly had shown links to Zika infection.
Microcephaly can be caused by a range of factors, including genetic conditions, infections and drugs.
Experts say women who are pregnant are most at risk from mosquito-borne Zika and should try to protect themselves from mosquito bites.