Brazil on Wednesday reported a new total of 508 confirmed cases of microcephaly, the serious birth defect suspected of being linked to the Zika virus -- a 10 percent jump in less than a week.

The latest figure was up from 462 confirmed cases on Friday, the health ministry said.

Another 3,935 suspected cases of microcephaly, in which a baby is born with an abnormally small head and often incomplete brain development, are being investigated.

Of those, nearly 40 percent came to light over the last six weeks, illustrating the rapid spread of a usually rare health problem.

Brazil usually reports about 150 cases of microcephaly annually. Only two of the country's 27 states have not reported cases.

Scientists in Brazil say the increase in microcephaly is linked to an explosion of the mosquito-transmitted Zika virus, with an estimated 1.5 million people infected.

The virus usually causes mild flu-like symptoms, but scientists say it might cause microcephaly in the fetuses of women infected while pregnant.

The World Health Organization is studying the possible link and calls the Zika outbreak an international health emergency.

In the absence of a vaccine against Zika, Brazil is mounting a huge operation to eradicate the mosquito responsible for carrying the virus.

Tens of thousands of troops have been called up to help health workers educate the public and eliminate pools of stagnant water where mosquitoes breed.

"The mosquito is completely domestic. It can be born, grow and live inside houses," health worker Paulo Lopez said as he fumigated residences in a suburb of the capital Brasilia on Wednesday.

With less than six months to go before the Olympic Games kick off in Rio, Brazil is under pressure to reassure international travelers and athletes that they run no risk from the little-understood virus.