The Zika virus is now a global emergency as the mosquito-borne disease spreads quickly, the World Health Organization declared Monday.

WHO Director-General Margaret Chan said international coordinated response was needed, although restrictions on travel or trade were not necessary.

Dr Thomas Frieden of the Centers for Disease Control said the declaration “calls the world to action” on Zika, The Telegraph reported.

Related: No need to panic over first case of Zika virus in Mass, say experts

The emergency designation was recommended by a committee of independent experts to the United Nations agency following criticism of a hesitant response so far. The move should help fast-track international action and research priorities.

The WHO said last week the Zika virus was “spreading explosively” and could infect as many as 4 million people in the Americas, Reuters reported.

Zika, first identified in Uganda in 1947 and unknown in the Americas until discovered in Brazil last year, causes a mild fever and body aches, symptoms that disappear in five days, though Brazil's top health official said on Monday that the virus outbreak is proving to be worse than believed because most cases show no symptoms.

"Eighty percent of the people infected by Zika do not develop significant symptoms. A large number of people have the virus with no symptoms, so the situation is more serious that we can imagine," Health Minister Marcelo Castro said in an interview with Reuters.

Improved testing should allow Brazil to get a better grip on the epidemic, Castro said.

The U.S. Food and Drug Administration, however, has said it is planning to require people who have traveled to an affected country to defer giving blood, but details on how that might work are still being determined.

-Reuters contributed to this report