The World Health Organization (WHO) expects the Zika virus, which is spreading through the Americas, to affect between three million and four million people, a disease expert said on Thursday.

The WHO's director-general said the spread of the mosquito-borne disease had gone from a mild threat to one of alarming proportions.

Marcos Espinal, an infectious disease expert at the WHO's Americas regional office, said: "We can expect 3 to 4 million cases of Zika virus disease". He gave no time frame.

There is no vaccine or treatment for Zika, which is a close cousin of dengue and chikungunya and causes mild fever, rash and red eyes. An estimated 80 percent of people infected have no symptoms, making it difficult for pregnant women to know whether they have been infected.

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WHO Director-General Margaret Chan said the organization's will convene an emergency committee on Monday to help determine the level of the international response to an outbreak of the virus spreading from Brazil that is believed to be linked to severe birth defects.

"The level of alarm is extremely high," Chan told WHO executive board members at a meeting in Geneva. "As of today, cases have been reported in 23 countries and territories in the (Americas) region."

Brazil's Health Ministry said in November 2015 that Zika was linked to a fetal deformation known as microcephaly, in which infants are born with abnormally small heads

Brazil has reported 3,893 suspected cases of microcephaly, the WHO said last week, more than 30 times more than in any year since 2010 and equivalent to 1-2 percent of all newborns in the state of Pernambuco, one of the worst-hit areas.

Chan said that while a direct causal relationship between Zika virus infection and birth malformations has not yet been established, it is "strongly suspected".

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