GENEVA (AP) – The World Health Organization has announced that the explosive spread of the Zika virus in the Americas is an “extraordinary event” that merits being declared an international emergency.

The agency convened an emergency meeting of independent experts on Monday to assess the outbreak after noting a suspicious link between Zika’s arrival in Brazil last year and a surge in the number of babies born with abnormally small heads.

Although WHO Director-General Dr. Margaret Chan said there was no definitive proof that the Zika virus, spread by mosquitoes, is responsible for the birth defects, she acknowledged on Thursday that “the level of alarm is extremely high.”

The last such public health emergency was declared for the devastating 2014 Ebola outbreak in West Africa, which killed more than 11,000 people.

WHO estimates there could be up to 4 million cases of Zika in the Americas in the next year.