The World Health Organization yesterday declared the mosquito-borne Zika virus to be an international public health emergency as the disease linked to thousands of birth defects in Brazil spreads rapidly.

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

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 four million people in the Americas.

The agency was criticised 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 WHO's International Health Regulations emergency committee brings together experts in epidemiology, public health and infectious diseases from the Americas, Europe, Asia and Africa.

Brazil has reported nearly 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.

Brazilian Health Minister Marcelo Castro told Reuters that the epidemic was worse than believed because the infected people had no symptoms in 80 percent of the cases.

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.

The Pan American Health Organization says that Zika has now spread in 24 nations and territories in the Americas.