The World Health Organisation is convening an emergency committee on Monday to decide if the Zika virus outbreak should be declared an international health emergency.

At a special meeting, WHO director-general Dr Margaret Chan said the virus - which has been linked to birth defects and neurological problems - is "spreading explosively".

Dr Chan said although there was no definitive proof that Zika was responsible for a spike in the number of babies being born with abnormally small heads in Brazil, "the level of alarm is extremely high".

The WHO last declared an international emergency over the devastating 2014 Ebola outbreak in West Africa, which ended up killing more than 11,000 people.

The Zika virus was first detected in 1947 and for decades only caused mild disease.

But Dr Chan noted that "the situation today is dramatically different".

Marcos Espinal, head of communicable diseases at PAHO -- the WHO's Americas arm -- said a study would soon be published suggesting a correlation between Zika and microcephaly -- babies born with small heads and brains -- in Brazil.

"We don't know yet if this virus crosses the placenta and generates or causes microcephaly. We think it plays a role. There's no doubt about that," he told the WHO's executive board.

He said the virus may infect 3-4 million people in the Americas, including 1.5 million in Brazil

According to the US Centres for Disease Control, the virus is now in more than 20 countries, mostly in Central and South America.

Dr Chan also noted a possible relationship between Zika infection and Guillain-Barre syndrome, which can cause temporary paralysis.

"The possible links, only recently suspected, have rapidly changed the risk profile of Zika from a mild threat to one of alarming proportions," Dr Chan said.

There is no specific treatment or vaccine for Zika, which is related to dengue - scientists have struggled for years to develop a dengue vaccine but have failed to create a viable shot so far.