Signs of Zika have been found in brain tissue of microcephaly babies
The Zika virus, which has swept through large parts of South America, causes microcephaly and other severe birth defects, US scientists have confirmed.
There have been growing fears the mosquito-borne disease was linked to such conditions, including in the worst-hit country Brazil.
But until now, many experts have been cautious about making a definitive connection.
This was despite a surge in the number of cases of Zika in pregnant women, and babies being born with abnormally small heads and brains - the microcephaly condition.
Dr Tom Frieden, director of the US Centers for Disease Control and Prevention (CDC), said there was enough evidence for the confirmed finding.
"There is no longer any doubt that Zika causes microcephaly," he said.
Dr Frieden pointed out that never before in history has a bite from a mosquito been seen as the cause of birth defects.
Signs of Zika have been found in the brain tissue, spinal fluid and amniotic fluid of microcephaly babies.
The confirmation should help officials make a more convincing case to the public to take precautions.
The virus is spread primarily through mosquito bites but can also be transmitted through sex.
It has also been associated with Guillain-Barre syndrome, a rare disorder in which the body's immune system attacks part of the nervous system.
CDC officials have been warning pregnant women to use mosquito repellent, avoid travelling to Zika-stricken regions and either abstain from sex or rely on condoms. Those guidelines will not change.
There have been at least 1,113 confirmed cases of microcephaly in Brazil since October.
There are fears the problem is only going to get worse in South America, the Caribbean and North America with the onset of mosquito season this spring and summer.
In February, the World Health Organisation (WHO) declared the outbreak an international emergency.
It said the spread of the Zika virus in South America was an "extraordinary event" and there could be up to four million cases in the region this year.
Brazil is due to host the Olympics this summer and pregnant women have been urged not to attend due to the risk of foetal brain damage.