Such emergency declarations are meant as an international SOS signal and usually trigger increased money and efforts to stop the outbreak, as well as prompting research into possible treatments and vaccines.

WHO officials say it could be six to nine months before science proves or disproves any connection between Zika and the spike in the number of babies born in Brazil with abnormally small heads.

WHO, which was widely criticized for its slow 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 August, when nearly 1,000 people had died.

Its officials say that up to 4 million cases of Zika could turn up in the Americas within the next year. Zika was first identified in 1947 in a Ugandan forest but until last year, it wasn't believed to cause any serious effects; about 80 percent of infected people never experience symptoms. The virus has also been linked to Guillain-Barre syndrome, which causes muscle weakness and nerve problems.

"Of course, the world and the World Health Organization have all learned from the Ebola crisis," WHO spokesman Christian Lindmeier said earlier Monday before the emergencywas declared. "That's why we are trying to bring in the best experts we can gather for this event, to try to establish what steps to take and what the way forward should be."

Lindmeier credited authorities in Brazil for being "extremely transparent" since the Zikaoutbreak turned up there in May. He said WHO first raised the possible connection between the virus and abnormally small heads back in October — a prospect that has sown fear among many would-be mothers and pregnant women.

Brazilian officials shared lab samples with foreign experts and brought in scientists from abroad, he said.

"What we know so far is that the only microcephaly cases we see currently are from Brazil," Lindmeier said, noting that abnormally small heads in newborns can have many causes — such as the effects of herbicides, alcohol use, or drugs and toxins. "This is exactly what is the concerning question: why do we see this in Brazil?"

Jimmy Whitworth, an infectious diseases expert at the London School of Hygiene and Tropical Medicine, said we might soon see other babies born with malformed heads as the virus becomes entrenched in other countries.

"It could be that we're getting the strongest signal in Brazil," he said before WHO's annoucement. "But having these cases occurring and pinning it to Zika is tough."

Whitworth said it was important for WHO to act quickly, despite definitive evidence thatZika is responsible for the surge in microcephaly cases.

"For situations like this, you have to essentially have a 'no regrets' policy," he said. "Maybe this will be a false alarm when more information is available months later, but it's serious enough on the evidence we have right now that we have to act."