The Zika virus, which has been blamed for a surge in birth defects in Central and South America, is "spreading explosively", the World Health Organisation’s chief has said.

The WHO says the virus could infect as many as 4 million people in the Americas.

WHO Director General Margaret Chan has called a emergency meeting for next Monday, 1 February, to determine if the outbreak qualifies as an international public health emergency.

"The level of alarm is extremely high," Ms Chan told a meeting of WHO member states in Geneva.

Ms Chan said that during previous outbreaks the virus, which was first discovered in a monkey in Uganda in 1947, "occasionally caused a mild disease of low concern."

But "the situation today is dramatically different," she said, highlighting the growing concern that Zika has links to a birth defect known as microcephaly, or an abnormally small head.

The WHO said it expects three to four million cases of the Zika virus in the Americas, as fears mount over the rapid spread of the disease.

"A causal relationship between Zika virus infection and birth malformations and neurological syndromes has not yet been established, but is strongly suspected," Ms Chan said.

She explained that the emergency committee meeting next week will seek "advice on the appropriate level of international concern and for recommended measures that should be undertaken in affected countries and elsewhere."