The World Health Organization (WHO) has set up a Zika "emergency team" after the "explosive" spread of the virus, the BBC reported Thursday.

Most people have no symptoms, but WHO expects "three to four million cases of Zika virus disease" in the Americas.

WHO director general Margaret Chan said Zika had gone "from a mild threat to one of alarming proportions" and was having a "heart-breaking" impact.

Zika was first detected in Uganda in 1947, but has never caused an outbreak on this scale.

Brazil reported the first cases of Zika in South America in May 2015.

Most cases result in no symptoms and it is hard to test for, but WHO officials said an estimated 1.5 million people had been infected in the country.

The virus, which is spread by mosquitoes, has since spread to more than 20 countries in the region.