The first case of birth defect microcephaly linked to the mosquito-borne Zika virus has appeared in Colombia, a doctor said on Friday, although the national health institute said it had no information on the case and could not confirm it.

Much remains unknown about Zika, including whether the virus actually causes microcephaly, a condition defined by unusually small heads that can result in developmental problems.

Colombia, seen as a key test case of the impact of the virus, has 42,706 cases of Zika, including 7,653 pregnant women.

A study of 28 women in Colombia's Sucre province infected with Zika during pregnancy has so far yielded one baby with microcephaly, said Alfonso Rodriguez-Morales, a doctor and researcher at the Technical University of Pereira. It was not immediately known when the child was born.

"The only infection that would explain what is happening is the Zika virus," he said.

One case does not prove an overall link between the virus and microcephaly, Rodriguez-Morales told Reuters in a phone interview, but his team has ruled out other potential causes of the defect in this child, including rubella, herpes, syphilis and toxoplasmosis.

The two other babies born to women in the study had cranial defects that are being investigated and could not so far be linked to Zika, Rodriguez-Morales said.

The virus is present in all three babies and the women, whose infections have been confirmed by lab tests, remain under observation.

The country's national health institute said it could not confirm the case because no samples from the patients had been sent to its laboratories. The institute currently is monitoring 28 children with potential microcephaly, not all related to Zika, but so far no case of the defect is linked to the virus, the institute told Reuters.

Rodriguez-Morales said researchers have sent the samples to the institute. The potential case was first reported by science journal Nature.

"We'll really see the impacts of Zika on newborns and pregnancy during the next two or three months," Rodriguez-Morales said. "One will be able to see the real magnitude of the problem."

Colombian health officials last week reported a "probable" case of microcephaly possibly linked to Zika in an aborted fetus.

The health minister has said original estimates of microcephaly cases may be too high.

Brazil said it has confirmed more than 640 cases of microcephaly, and considers most of them to be related to Zika infections in the mothers. It is investigating more than 4,200 additional suspected cases.

(Reporting by Julia Symmes Cobb; Editing by Bill Trott)