Brazil's Health Ministry said Wednesday that in recent months it has recorded 4,120 suspected cases of a rare brain defect in babies that officials fear may be linked to the spread of the mosquito-borne Zika virus, according to AP.

But so far, only 270 of the cases have been confirmed by lab tests as microcephaly involving brain damage, and the defect was ruled out in 462 cases, the ministry said. Researchers are still investigating 3,448 of the cases, which were recorded from Oct. 22 to Jan. 23.

The ministry said laboratories are trying to determine a link between the Zika virus and microcephaly, which also can be caused by factors such as herpes, rubella and syphilis. The rare birth defect causes babies to be born with unusually small heads and can cause lasting developmental problems.

Brazilian health officials estimate they had 150 cases of microcephaly in all of 2014 and the surge in suspected cases and the possible link to Zika have caused worries across Latin America's biggest nation and in other countries in the Western Hemisphere.

On Tuesday, Brazil's health minister, Marcelo Castro, announced that 220,000 military personnel were being deployed to bolster efforts to eradicate the Aedes aegypti mosquito that transmits Zika, dengue, chikungunya and yellow fever.

The World Health Organization has stressed that a link remains circumstantial and is not yet proven scientifically.

The U.S. Centers for Disease Control and Prevention is advising pregnant women to reconsider travel to Brazil and 21 other countries and territories with Zika outbreaks. Officials in El Salvador, Colombia and Brazil have suggested women stop getting pregnant until the crisis has passed.