The World Health Organization on Monday said that severe complications associated with the spread of the Zika virus constitute a global public health emergency, a move that could help officials generate more resources to respond to the outbreak.

The declaration serves as an international alarm on the spread of the virus, which has been linked to a spike in birth defects. No recommendations were made to restrict travel or trade, however.

“We need a coordinated international response to make sure we get to the bottom of this,’’ the WHO’s director general, Margaret Chan, said in a news conference at the agency’s headquarters in Geneva.

The outbreak of mosquito-borne Zika began last May in Brazil, and WHO now estimates there could be up to 4 million cases of the virus in the Americas in the next year. Several dozen cases have been reported in the United States among travelers — including one from the Boston area — who have returned from countries where there is active Zika transmission.

On its own, Zika causes mostly mild flu-like symptoms and would not have been deemed a global health emergency, officials said Monday. But a WHO panel of 18 outside experts advising the agency concluded that a coordinated global response was necessary given the suspected link between Zika and a rapid increase in two serious conditions: a severe birth defect called microcephaly, in which newborns have abnormally small heads, and Guillain-Barré syndrome, a neurological condition that triggers temporary paralysis.

Officials theorize that infection during pregnancy could induce microcephaly in the fetus. Brazil has reported more than 4,000 suspected microcephaly cases since October. It typically sees about 175 cases annually.

Brazil and a few other Latin American countries have also reported more modest increases in cases of Guillain-Barré syndrome. People who develop it generally recover fully, although it can be fatal.

The announcement marked the fourth public health emergency of Chan’s tenure, which began in 2007. Emergencies were also called to deal with the 2009 H1N1 flu pandemic, West Africa’s Ebola outbreak, and problems plaguing the polio eradication campaign.

The announcement puts the WHO, which was widely condemned for its slow response to the Ebola epidemic, in a leadership role as public health officials seek to better understand the connection between Zika and birth defects, among other issues.

The emergency committee on Zika recommended that the WHO call for increased surveillance for microcephaly and Guillain-Barré syndrome and that the definitions used to diagnose both be standardized, so that all countries are using the same yardstick when assessing the incidence of such cases.

The committee called for rapid development of Zika virus tests and vaccines — though it listed the latter as a longer-term measure. It also sought enhanced communications aimed at women in affected countries who may become pregnant and strengthened mosquito control measures.

The committee did not recommend — and the WHO did not issue — travel advice for pregnant women. Chan said the committee found no public health justification for restrictions on travel or trade.

The US Centers for Disease Control and Prevention, by contrast, has urged women who are pregnant to consider avoiding travel to more than two dozen countries and territories experiencing Zika outbreaks.

On Monday, the agency added American Samoa, Costa Rica, Curacao, and Nicaragua to its list.

Some public health experts said the WHO’s decision not to issue travel guidance was a mistake.

“Parents concerned for the welfare of their daughters would advise them not to travel to affected areas if they are pregnant,’’ said Lawrence Gostin, faculty director of the O’Neill Institute for National and Global Health Law at Georgetown University in Washington.

Gostin, who had been publicly pushing the WHO to declare the Zika outbreak a global health emergency, noted that the CDC has also advised women considering becoming pregnant to talk with their doctor before traveling.

“That is the right thing to do from a public health and ethical perspective,’’ he said.

The Zika virus, once thought to be a weak cousin of the more severe dengue and chikungunya viruses, has gained global attention in recent weeks since Brazil began to voice fears that the country was experiencing a surge in cases of babies born with microcephaly.

Several affected countries in Latin America are recommending that women delay pregnancy — advice that advocacy groups have denounced as unworkable in countries where access to contraceptives may be limited and where abortions are outlawed.

The Zika virus was first identified in 1947 in the Zika Forest of Uganda. It has been studied little, however, because it was not seen as a widespread threat to humans.

Four out of five people infected show no symptoms, and those who do experience something like the flu — fever and achy muscles and joints.

People who contract Zika may also develop a raised red rash and/or conjunctivitis, commonly known as pink eye.

Material from the Associated Press was used in this report. Helen Branswell can be reached at helen.branswell@statnews. com. Follow Helen on Twitter @HelenBranswell