World Health Organisation chief Margaret Chan said on Thursday that the mosquito-borne Zika virus is “spreading explosively”. She added that the level of alarm is extremely high, and called for a meeting on February 1 to determine if the outbreak qualifies as an international public health emergency. It was “deeply concerning” that the virus has now been detected in 23 countries in the Americas, she added.

Three to four million cases of Zika can be expected in the Americas, according to another WHO official, though he gave no time frame for these figures. The virus is blamed for a surge in birth defects in Latin America, reported AFP. Since September, Brazil has registered nearly 4,000 cases of babies suffering from microcephaly, a serious condition linked to Zika that can cause lifelong developmental problems.

The spread of the virus has prompted governments across the world to advise pregnant women against going to the areas where it has been detected, reported The Guardian. There is no vaccine or cure for Zika.