GENEVA — The World Health Organization declared a global emergency over the explosive spread of the Zika virus, which has been linked to birth defects in the Americas, calling it an “extraordinary event” that poses a public health threat to other parts of the world.

The U.N. agency took the rare step despite a lack of definitive evidence proving the mosquito-borne virus is causing a surge in babies born with brain defects and abnormally small heads in Brazil and following a 2013-14 outbreak in French Polynesia.

Monday’s emergency meeting of independent experts was called in response to the spike in babies born with microcephaly in Brazil since the virus was first found there last year. Officials in French Polynesia also documented a connection between Zika and neurological complications when the virus was spreading there two years ago, at the same time as dengue fever.

“After a review of the evidence, the committee advised that the clusters of microcephaly and other neurological complications constitute an extraordinary event and public health threat to other parts of the world,” WHO Director-General Dr. Margaret Chan said.

WHO, which was widely criticized for its sluggish response to the 2014 Ebola crisis in West Africa, has been eager to show its responsiveness this time. Despite dire warnings that Ebola was out of control in mid-2014, WHO didn’t declare an emergency until months later, after nearly 1,000 people had died.