The World Health Organization on Monday declared that a rise in the number of birth defects and other neurological conditions possibly linked to the Zika virus constitutes a global public health emergency.

The United Nations public health agency, acting on the recommendations of an emergency committee, called for more surveillance, research, and efforts to control the virus’s spread. It also pushed for the development of more readily available tests to diagnose the virus—none are commercially available now—as well as drugs and vaccines.

“The evidence is growing and it’s getting strong,” WHO Director-General Margaret Chan said at a news conference, of the suspected link between Zika and the birth defect, known as microcephaly, in which babies are born with undersized heads and brains. “We need a coordinated international response to make sure we get to the bottom of this.”

Dr. Chan convened the emergency committee last week, when she declared that Zika was spreading “explosively.”

The agency’s decision shows how a once-obscure virus, which generally makes people only mildly ill for a few days, has become a major global health concern as it has spread first in Brazil and now throughout the Americas. Brazil health authorities believe as many as 1.5 million Brazilians may be infected with the Zika virus.

The WHO said it declared the Zika epidemic an emergency only because of its possible links to microcephaly and Guillain-Barré, a rare disorder in which the body’s immune system attacks nerve cells.

“Zika alone would not be a public health emergency of international concern,” said David Heymann, the committee’s chairman and professor of infectious disease epidemiology at the London School of Hygiene and Tropical Medicine. “Zika is not a clinically serious infection. It’s only because of this association, if it’s proven.”

The WHO also said travel and trade shouldn’t be restricted due to Zika. That put it seemingly at odds with the U.S. Centers for Disease Control and Prevention, which added four more countries and territories to a growing list, now totaling 28, of places to which it has warned pregnant women to avoid travel because Zika is circulating there. But those alerts don’t constitute a travel ban.

The WHO’s decision followed a four-hour teleconference Monday afternoon Geneva time with the 12-member emergency committee and a team of advisers, including medical experts and representatives of the tourism and aviation industries. The governments of Brazil, El Salvador, France and the U.S. provided their findings on the possible link between Zika and microcephaly and other neurological disorders, the WHO said.

Brazil’s health ministry has confirmed 270 cases of microcephaly since October, when observations by doctors in the northeastern city of Recife led to the country mandating the reporting of suspected cases. Of those, six cases have confirmed links to the Zika virus. The Brazil health ministry is investigating thousands more suspected cases of microcephaly.

Brazil isn’t alone. Authorities now say they believe that some cases of microcephaly in babies in French Polynesia may be linked to an outbreak of Zika that occurred there in 2013 and 2014. Brazilian and international health authorities also say Zika may be linked to a rise in the number of cases of Guillain-Barré syndrome. Cases are being investigated in Brazil, El Salvador, and French Polynesia.

The emergency committee called for more surveillance for microcephaly and GBS and research into whether there is a causal link with the rise in Zika cases. It also called for more surveillance for Zika virus itself, and measures to combat the mosquitoes that spread it—primarily the Aedes aegypti species, which is common in the tropics and subtropics, including the southern U.S.

The species is considered a formidable threat to public health because it is an aggressive biter, hard to eliminate, and it spreads dengue and chikungunya, which exact an even harsher toll on many of their victims than the normally mild Zika virus.

The committee said women of childbearing age should be taught how to reduce exposure to Zika, and those who are pregnant should be counseled and monitored.

Declarations of global health emergencies are a way for the WHO to mobilize resources and coordinate a response, said Lawrence Gostin, an expert in public health law at Georgetown University Law Center. He praised the WHO Monday for sounding the global alert, given Zika’s spread and the fact that very few people have developed immunity to it.

But, he said, now the WHO, which is strapped for resources of its own, must follow up the declaration with actions. “They can’t just sound an alarm—they need to coordinate a global response with funding and I don’t see the strategy yet to do that,” said Dr. Gostin, who publicly urged the WHO last week to declare Zika a public health emergency.

The WHO’s definition of the emergency—a birth defect and neurological conditions rather than Zika itself—was too narrow, he added. “It signals that Zika itself is not the problem, whereas Zika may very well be the problem,” he said. “It also means there will be less of an emphasis on what needs to be done, which is declaring war on this species of mosquitoes.”

Monday’s declaration suggests the WHO is becoming more aggressive in its response to epidemics, after it was heavily criticized for declaring Ebola to be a public health emergency months into that epidemic, when it was already spinning out of control.

This is only the fourth time the WHO has declared the spread of a disease to be a “public health emergency of international concern,” a designation introduced to indicate international risk and corral political and financial support to fight it. Emergencies were declared previously for the H1N1 flu pandemic in 2009, the spread of polio in 2014, and Ebola later the same year.

But the agency held off on declaring emergencies for other recent epidemics, including Middle East Respiratory syndrome, which has infected at least 1,633 people in 26 countries, killing 587 of them, since 2012.

“The World Health Organization was widely criticized in the recent past for not quickly declaring an emergency in the case of Ebola,” said Carlos Fortaleza, an infectious disease specialist at São Paulo state university, UNESP. “With Zika they were rapid, and they are correct. This decision opens space for Brazil to obtain more international assistance in the areas of research, for the control of the [virus transmission] vector, and also possibly for faster production of a vaccine.”

The agency has called the Ebola epidemic unprecedented, and said it is making several changes to help it respond better to epidemics.

Many leading medical authorities agree that extensive additional testing, likely lasting six months or more, must be done to determine whether a link exists between Zika and microcephaly and Guillain-Barré. Researchers are combing through medical records and studying cohorts of pregnant women in Brazil, El Salvador and elsewhere. Several studies are under way, the WHO said, and more would begin soon.