Experts have found "extremely severe" brain damage in babies born with a birth defect presumed to have been caused by the Zika virus
.Doctors in Brazil have mapped out the largest set of brain scans of children with microcephaly which was presumably caused by mothers becoming infected with the virus while pregnant.
They examined 23 youngsters and found "severe cerebral damage" which indicates a "poor prognosis for neurological function".
Microcephaly is a rare birth defect where a baby is born with an unusually small head.
Since October last year 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 BMJ, describes a range of brain abnormalities found in babies with microcephaly born in the Brazilian state of Pernambuco between July and December 2015.
All but one of the babies were born to mothers who had a rash during pregnancy, consistent with a Zika virus 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 neuroimaging 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.
Earlier this week US public health officials said that the Zika virus was "scarier" than first thought.
Dr Anne Schuchat of the US Centers for Disease Control and Prevention (CDC) said: "Everything we know about this virus seems to be scarier than we initially thought."
There has been active transmission of the virus 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 current outbreak, 12 British travellers are known to have been infected with the virus.
Most people who are infected have no symptoms at all, but some infected individuals experience mild flu-like symptoms and skin rashes.