The UN's health agency warned on Monday of the potential for a "marked increase" in Zika infections, and the spread of the virus to new parts of the world, even as the outbreak declines in Brazil.
Largely contained to Latin America and the Caribbean, Zika's range is likely to expand as summer arrives in the northern hemisphere - and with it virus-transmitting mosquitoes.
"As seasonal temperatures begin to rise in Europe, two species of Aedes mosquito which we know transmit the virus will begin to circulate," World Health Organisation assistant director general Marie-Paule Kieny told a Zika science conference in Paris.
"The mosquito knows no borders." Add to that the risk of Zika-infected men passing the virus on to women via sex, and the world "could see a marked increase in the number of people with Zika and related complications," Kieny said.
At the same time, with cooler temperatures in the tropics and subtropics, the outbreak in hardest-hit Brazil was "clearly on the decline," she added, without providing numbers.
About 600 disease experts from 43 nations are in the French capital to pore over scant but increasingly worrisome data about Zika.
What we don't know 'a concern'
Despite a flurry of research, very little is known about the virus - how long it can hide out in the human body, the degree of risk of sexual transmission, the full list of diseases and disorders it may cause, and all the mosquito types capable of transmitting it.
Recent scientific consensus is that Zika causes microcephaly, a form of severe brain damage in new-borns, and adult-onset neurological problems such as Guillain-Barre Syndrome, which can cause paralysis and death.
"It's not what we know but what we don't know that is concerning," infectious diseases professor David Heymann of the London School of Hygiene and Tropical Medicine told AFP on the conference sidelines.
Kieny described Zika as a "global emergency" and a "growing threat".
There is no vaccine or treatment for the virus, which in most people causes only mild symptoms - a rash, joint pain or fever.
The most urgent priority, Kieny said, was for new tools for quickly diagnosing Zika - particularly in pregnant women whose babies risk severe disability.
Developers in the United States, France, Brazil, India and Austria are working on 23 vaccine-development projects, Kieny said. But it could take years, and the feasibility of an "emergency-use" vaccine was being examined.
Until then, the first line of defence remains mosquito control and bite prevention, the experts said, and advising women in endemic regions to put off pregnancy.
Duane Gubler of the Duke-NUS Medical School in Singapore, said Zika "surprised" the world, just as Ebola before it, despite both viruses having been known about for decades.
"I think we should take this as a wakeup call and start developing our surveillance systems so we can monitor these viruses a little more effectively," he told the conference.
Kieny said particular vigilance was required in Africa, where the virus was first discovered in Uganda in 1947.
For Europe, the risk is somewhat lowered by the fact that the mosquito Aedes albopictus present in 20 countries in summer, is less "prone" to causing outbreaks than its cousin A. aegypti in the tropics, according to new research unveiled by the Pasteur Institute.
While cases of local transmission are possible, the risk of a full-blown European outbreak "appears low", French immunologist Jean-Francois Delfraissy said.