GENEVA/CHICAGO • Researchers around the world are now 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 World Health Organisation (WHO) said.

The statement represented the United Nations health agency's strongest language to date on the link between the mosquito-borne virus and the two maladies.

The WHO also 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 is 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 represents 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 on Thursday.

In its previous weekly report, the WHO had said Zika was "highly likely" to be a cause.

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 sizes that can result in severe developmental problems.

Brazil's health department this week reported 944 confirmed cases of microcephaly, and most are 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 in Houston, Texas, was not surprised by the WHO's statement. "The evidence is just so overwhelming," he said.

REUTERS