The designation would fast-track international action and research priorities, following criticism of a hesitant response so far.
The United Nations agency said last week the Zika virus was "spreading explosively" and could infect as many as 4 million people in the Americas.
The WHO was criticized for reacting too slowly to the Ebola epidemic in West Africa which killed more than 10,000 people, and has promised to do better in future global health crises. The 12 committee members, who are experts in epidemiology, public health and infectious diseases from the Americas, Europe, Asia and Africa, discussed the issue in a telephone conference. A news briefing will be held on Tuesday afternoon at the earliest, the WHO said.
Peter Piot, director of the London School of Hygiene and Tropical Medicine and a co-discoverer of the Ebola virus, said the WHO had "dropped the ball" on Ebola, when it took five months for the epidemic to be declared an emergency.
He told BBC Radio he expected a rapid response this time. "I have all confidence that they will declare this as a public health emergency," he said.
Brazil has reported around 4,000 suspected cases of microcephaly, in which infants are born with smaller-than-usual brains. The health ministry has linked the condition to Zika, although the connection is not yet definitive. As the virus spreads from Brazil, other countries in the Americas are also likely to see cases of babies with Zika-linked birth defects, experts believe.