New Zealand and five other countries are reporting the spread of Zika virus - without it being carried by mosquitoes, the World Health Organisation (WHO) says.
It said the locally-acquired infections are probably through infected patients having un-protected sex.
These cases are being reported in New Zealand, Argentina, Chile, France, Italy, and the United States.
Sydney University epidemiologist Dr Grant Hill-Cawthorne said men travelling from Zika zones should now use condoms for the next six months in line with this advice.
Researchers around the world are now also convinced the Zika virus can cause the birth defect microcephaly as well as Guillain-Barre syndrome, a rare neurological disorder that can result in paralysis, the WHO said today.
The statement represented the UN health agency's strongest language to date on the connection between the mosquito-borne virus and the two maladies.
The researchers reported the first sign of a possible rise in microcephaly cases outside Brazil, the hardest-hit country so far in an outbreak spreading rapidly in Latin America and the Caribbean.
Neighbouring Colombia was investigating 32 cases of babies born with microcephaly since January, and eight of them so far have tested positive for the Zika virus, the WHO said.
This number of microcephaly cases reported in Colombia so far represented an increase over the historical annual average of about 140 cases.
"Based on observational, cohort and case-control studies, there is a strong scientific consensus that Zika virus is a cause of GBS (Guillain-Barre syndrome), microcephaly and other neurological disorders," the WHO said.
In its previous weekly report, the WHO had said Zika was "highly likely" to be a cause.
In February the WHO declared the Zika outbreak an international health emergency, and cited a "strongly suspected" relationship between Zika infection in pregnancy and microcephaly.
Although Zika has not been proven conclusively to cause microcephaly in babies, evidence of a link was based on a major spike in Brazil in cases of microcephaly, defined by unusually small head size that can result in severe developmental problems.
Brazil's health department this week reported 944 confirmed cases of microcephaly, and most were believed to be related to Zika infections in the mother.
Dr Peter Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine, said he was not surprised by the WHO's statement.
"The evidence is just so overwhelming."
The link to Guillain-Barre had also been pretty clear, he said.
"The only lack of clarity is the percentage of pregnant women infected with Zika who give birth to a baby with microcephaly."
This appeared to be much higher than what was seen in a previous outbreak in French Polynesia.
While Guillain-Barre was a concern, Hotez said, "the overwhelming emphasis needs to be on preventing microcephaly in babies".
In recent studies, researchers have seen evidence of the virus in brain cells of stillborn and aborted fetuses. They have also seen signs that the brain had been growing normally, but that growth was disrupted and the brain actually shrank.
Scientists have been closely monitoring for possible microcephaly cases outside Brazil to rule out environmental factors in Brazil as a cause. Colombia has been following the pregnancies of women infected with Zika after seeing widespread transmission of the virus since October.
To date, 13 countries or territories have reported increased incidence of Guillain-Barre or laboratory confirmation of a Zika virus infection in people with the rare autoimmune disease, it added.