Brazil urged its neighbors on Wednesday to unite in fighting the Zika virus, blamed for a surge in brain-damaged babies, as airlines offered refunds to pregnant mothers afraid to travel to the country.

Brazil has been the country hardest hit by the outbreak of the mosquito-borne virus, which is blamed for a sharp rise in infants born with microcephaly, or abnormally small heads.

The outbreak is particularly concerning officials as the country prepares to host the Olympic Games, which will bring hundreds of thousands of travelers from around the world to Rio de Janeiro in August.