Scientists have found the Zika virus in the amniotic fluid of two pregnant women whose foetuses have been diagnosed with a congenital brain condition.
Earlier this month the World Health Organisation (WHO) said the link between microcephaly found in babies born to infected mothers should be considered a "public health emergency of international concern".
While the link has not yet been scientifically proven, the latest research suggests that the virus can cross the placental barrier.
So far 36 countries have been affected by the outbreak and WHO officials have predicted as many as four million people could be infected with the virus this year.
In 2015, the number of reported cases of babies with microcephaly in Brazil increased 20-fold when compared with previous years.
Babies born with microcephaly have abnormally small heads and are at risk of incomplete brain development.
The latest report, published in The Lancet Infectious Diseases journal, details two cases of women aged 27 and 35 from north-east Brazil. The women presented with symptoms of Zika infection during the first trimester of their pregnancies and ultrasounds confirmed the foetuses had microcephaly.
Samples of a mniotic fluid, the protective liquid around the foetus contained by the amniotic sac, were analysed for infections and scientists confirmed the presence of the virus.
The report's lead author Dr Ana de Filippis, from the Oswaldo Cruz Institute in Rio de Janeiro, Brazil, said: " Previous studies have identified Zika virus in the saliva, breast milk and urine of mothers and their newborn babies, after having given birth. This study reports details of the Zika virus being identified directly in the amniotic fluid of a woman during her pregnancy, suggesting that the virus could cross the placental barrier and potentially infect the foetus.
"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 virus to microcephaly we cannot be certain that one causes the other, and further research is urgently needed."
Commenting on the report, Jimmy Whitworth, professor of international public health at the London School of Hygiene and Tropical Medicine, said: "This study does strengthen the body of evidence that Zika virus is the cause of foetal microcephaly in Brazil. The investigators have shown that the virus crossed the placental barrier during the first and second trimesters of pregnancy in two women with fever and rash.
"Importantly they did not find evidence of any other of the main infections known to cause microcephaly. Studies of this sort can show an association between Zika and microcephaly but cannot show that Zika virus definitely caused the microcephaly.
"This will require the accumulation of evidence from a variety of studies from different perspectives. Questions that we urgently need to answer include what is the added risk of microcephaly if a woman has Zika virus infection in pregnancy, is the timing of infection during pregnancy important, and does Zika virus infection alone cause this birth defect or is a co-factor involved."
So far seven Britons are known to have acquired the Zika virus while travelling abroad - one of the cases is not associated with the current outbreak.