US health officials have confirmed that the Zika virus causes babies to be born with abnormally small heads and severe brain defects, confirming the worst fears of many pregnant women in the US and Latin America.

Doctors in Brazil have been linking Zika infections in pregnant women to a rise in newborns with microcephaly - an unusually small skull - since last year.

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Most outside experts were cautious about drawing such a connection but the US Centers for Disease Control and Prevention says it has enough evidence to confirm that.

"There is no longer any doubt that Zika causes microcephaly," Tom Frieden, CDC director, said on Wednesday.

The CDC said it also is clear that Zika causes other serious defects, including damaging calcium build-ups in the developing brain.

Among evidence that clinched the case was that signs of the Zika virus, which is spread primarily through mosquito bites and can also be transmitted through sex, have been found in the brain tissue, spinal fluid and amniotic fluid of microcephaly babies.

The CDC and other health agencies have been operating for months on the assumption that Zika causes brain defects, and they have been warning pregnant women to use mosquito repellent, to cover up, to avoid travel to Zika-stricken regions and either to abstain from sex or to rely on condoms.

Those guidelines will not change.

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But the new finding should help officials to make a more convincing case to the public for taking precautions.

Some experts hope it will change public thinking about Zika the way the 1964 surgeon-general's report convinced many Americans that smoking caused lung cancer.

"We've been very careful over the last few months to say 'it's linked to, it's associated with'," said CDC's Sonja Rasmussen.

"We've been careful to say it's not the cause of. I think our messages will now be more direct."

The World Health Organization made similar statements recently and an official applauded the CDC report.

"We feel it's time to move from precautionary language to more forceful language to get people to take action," said Bruce Aylward, who is leading WHO's Zika response.

Zika has been sweeping through Latin America and the Caribbean in recent months, and the fear is that it will only get worse there and arrive in the US with the onset of the mosquito season this spring and summer.

The virus causes only a mild and brief illness, at worst, in most people.

In the past year, infections in pregnant women have been strongly linked to fetal deaths and devastating birth defects, mostly in Brazil, where the health ministry said on Tuesday that 1,113 cases of microcephaly have been confirmed since October.