US health officials have said there is now enough evidence to say the Zika virus can cause unusually small heads and brain damage in babies.
The Centres for Disease Control and Prevention (CDC) said there was an established link between pregnant women catching Zika and their babies developing microcephaly as well as other neurological abnormalities.
Dr Thomas Frieden, from the CDC, said: "There is no longer any doubt that Zika causes microcephaly.
"Never before in history has there been a situation where a bite from a mosquito can result in a devastating malformation."
It comes as experts in Brazil said they had found evidence of "extremely severe" brain damage in babies.
They have mapped out the largest set of brain scans of children with microcephaly which was presumably caused by mothers being infected with the virus while pregnant.
They examined 23 youngsters and found "severe cerebral damage" which indicates a "poor prognosis for neurological function".
Since October there has been a significant increase in the number of cases of microcephaly among babies born in Brazil, which has led to scientists linking the condition with the virus.
In February, 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".
The new study, published in the British Medical Journal (BMJ), describes a range of brain abnormalities found in babies with microcephaly born in the Brazilian state of Pernambuco between July and December last year.
All but one of the babies were born to mothers who had a rash during pregnancy, consistent with a Zika infection. Other infectious causes of microcephaly, such as toxoplasmosis, cytomegalovirus, rubella, syphilis and HIV, were ruled out.
The team, led by Professor Maria de Fatima Vasco Aragao, analysed the types of abnormalities and lesions shown in MRI and CT brain scans of the children.
The scans revealed that the majority of babies had severe brain damage.
"This study shows the largest and most detailed case series of neuro-imaging findings in children with microcephaly and presumed Zika virus-related infection to date," the authors wrote.
"We have described the imaging (CT and MRI) findings in a series of children with presumed Zika virus-related congenital infection, which in most of the cases show severe cerebral damage.
"The brain damage caused by Zika virus infection in these children was extremely severe, indicating a poor prognosis for neurological function."
Researchers noted brain calcifications, a condition in which calcium builds up in the brain, and other problems including malformations of cortical development, decreased brain volume, and ventriculomegaly - a condition where the brain cavities are abnormally enlarged.
They also observed underdevelopment of the cerebellum and the brainstem.
There has been active transmission of Zika over the last nine months in countries in the Caribbean, Central America, South America, the Pacific and some of south-east Asia.
Since the start of the outbreak, 12 British travellers are known to have been infected with the virus.
Most people who are infected have no symptoms, but some experience mild flu-like symptoms and skin rashes.