Brazil has confirmed 641 cases of microcephaly and 139 babies with the birth defect who have died since the Zika virus outbreak started in October, the Health Ministry said.

The latest figure was up 10 percent over the number of microcephaly cases reported by the ministry last week.

Authorities were investigating another 4,222 possible cases of microcephaly, in which a baby is born with an abnormally small head and often incomplete brain development.

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

Brazil typically reports 150 cases of microcephaly per year.

The birth defect is also associated with mothers who contract syphilis, rubella or toxoplasmosis during pregnancy.