The Zika virus could infect up to four million people, the World Health Organisation has warned.

The virus, which is strongly suspected of causing birth defects including the shrinking of foetuses' brains and heads, has the potential to become an "explosive pandemic", WHO also said.

Marcos Espinal, head of communicable diseases at the WHO's Americas arm, said a study would soon be published suggesting a correlation between Zika and microcephaly 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 said.

A three-months-old, who has microcephaly, in Recife, Brazil. (Getty Images)

It comes as Brazilian experts at Oswaldo Cruz Foundation said the virus - thought to be confined to the Aedes Aegypti mosquito in the tropics - may have already crossed over to the culex mosquito - which would increase its chances of being spreading around the world

The culex is 20 times more common in Brazil and present in parts of Africa and Asia.

The Zika virus has been linked to the epidemic of microcephaly in Brazil and the Americas, which causes babies to be born with small heads with a circumference of less than 31.5-32 cm at birt

Constancia Ayres, research co-ordinator said: "The interaction of the mosquito with the virus may explain the epidemiological profile of disease transmission," reported the Telegraph.

A pediatric infectologist examines a two-months-old baby, who has microcephaly, on 26 January 2016 in Recife, Brazil. (Getty Images)

Ms Ayres added that if culex mosquitoes do carry the virus, "it means much more combined efforts from a lot of other countries are going to be needed to combat the virus".

US scientists have urged the World Health Organisation (WHO) to learn from the Ebola outbreak and take urgent action on combatting the virus, which they say has "explosive pandemic potential", and plan to organise an emergency committee of disease experts the BBC reported.

Battling the zika virus - in pictures Show all 19 1 /19 Battling the zika virus - in pictures Battling the zika virus - in pictures A worker of the Salvadorean Ministry of Health fumigates a house in Soyapango, 6 kilometers from San Salvador, El Salvador. Salvadorean authorities have began a three days campaign of fumigation to reduce the presence of the mosquito that transmit the Zika virus. EPA/Oscar Rivera Battling the zika virus - in pictures A Health Ministry employee fumigates a home against the Aedes aegypti mosquito to prevent the spread of the Zika virus in Soyapango, six km east of San Salvador. Health authorities have issued a national alert against the Aedes Aegypti mosquito, because of the link between the Zika virus and microcephaly and Guillain-BarrÈ Syndrome in foetuses. AFP PHOTO/Marvin RECINOSMarvin RECINOS/AFP/Getty Images Battling the zika virus - in pictures A pediatric infectologist examines a two-months-old baby, who has microcephaly, on 26 January 2016 in Recife, Brazil. Getty Images Battling the zika virus - in pictures A woman walks through the fumes as Health Ministry employee fumigate against the Aedes aegypti mosquito to prevent the spread of the Zika virus in Soyapango. Marvin RECINOS/AFP/Getty Images Battling the zika virus - in pictures A health ministry employee sprays to eliminate breeding sites of the Aedes Aegypti mosquito, which transmits diseases such as the dengue, chicunguna and Zica viruses, in a Tegucigalpa cemetery on January 21, 2016. The medical school at the National Autonomous University of Honduras (UNAH) recommended that women in the country avoid getting pregnant for the time being due to the presence of the Zika virus. If a pregnant woman is infected by the virus, the baby could be born with microcephaly. AFP PHOTO/Orlando SIERRA Battling the zika virus - in pictures A man walks away from his home with his son as health workers fumigates the Altos del Cerro neighbourhood as part of preventive measures against the Zika virus and other mosquito-borne diseases in Soyapango, El Salvador REUTERS/Jose Cabezas Battling the zika virus - in pictures A three-months-old, who has microcephaly, in Recife, Brazil. Getty Images Battling the zika virus - in pictures A pregnant woman waits to be attended at the Maternal and Children's Hospital in Tegucigalpa. The medical school at the National Autonomous University of Honduras (UNAH) recommended that women in the country avoid getting pregnant for the time being due to the presence of the Zika virus. If a pregnant woman is infected by the virus, the baby could be born with microcephaly. ORLANDO SIERRA/AFP/Getty Images Battling the zika virus - in pictures Army soldiers apply insect repellent as they prepare for a clean up operation against the Aedes aegypti mosquito, which is a vector for transmitting the Zika virus in Sao Paulo, Brazil. AP Photo/Andre Penner Battling the zika virus - in pictures Workers disinfect the Sambadrome in Rio de Janeiro to fight the spread of the Zika virus Battling the zika virus - in pictures Dr. Vanessa Van Der Linden, the neuro-pediatrician who first recognized the microcephaly crisis in Brazil, measures the head of a 2-month-old baby with microcephaly in Recife Battling the zika virus - in pictures Mother Mylene Helena Ferreira cares for her son David Henrique Ferreira, 5 months, who has microcephaly, on January 25, 2016 in Recife, Brazil. In the last four months, authorities have recorded close to 4,000 cases in Brazil in which the mosquito-borne Zika virus may have led to microcephaly in infants Getty Images Battling the zika virus - in pictures U.S. women who are pregnant from traveling to many South American countries Battling the zika virus - in pictures In the last four months, authorities have recorded close to 4,000 cases in Brazil in which the mosquito-borne Zika virus may have led to microcephaly in infants. Getty Images Battling the zika virus - in pictures Dr. Vanessa Van Der Linden, the neuro-pediatrician who first recognized the microcephaly crisis in Brazil, examines a two-month-old baby with microcephaly on January 27, 2016 in Recife, Brazil Battling the zika virus - in pictures Washington Post Battling the zika virus - in pictures Battling the zika virus - in pictures Battling the zika virus - in pictures

Authorities have reportedly put troops on the street and deployed health workers in a bid to combat the virus spreading further.

Dilma Roussef, Brazil’s president said at a summit in Ecuador that sharing knowledge between Latin American countries is the best way to combat the epidemic.

Already, thousands of people have been affected by the disease, which has spread across 20 countries since late 2015.

There are fears Aedes Aegypti mosquitoes have been transported to Britain in the tread of tyres, where females lay their eggs in trapped water, following several sightings.

A doctor measures the head of a baby with microcephaly in Brazil

Howard Carter, one of the country’s leading bite experts, has seen the mosquitoes, which can carry the virus, on the Kent coast and in West Sussex.

He added the mosquitoes are not in the UK in "any great number", but thinks it is "only a matter of time before that becomes the case", suggesting global warming is creating a warmer and more attractive climate in Britain, reports the Mail Online.

Brazil has been worst hit by the virus, with around a million people thought to be infected. People travelling to the Americas are warned to cover up to protect from bites and use a repellent with PMD in it, which is effective against the Aedes Aegypti mosquito.

A five-months-old baby, who has microcephaly, on 25 January 2016 in Recife, Brazil. (2016 Getty Images)

Three Britons have already contracted the disease after travelling to South and Central America, which was confirmed by Public Health England on Saturday.

Women in Latin America have been urged by the El Salvador government not to get pregnant for as long as two years, in a bid to avoid the birth defects associated with the virus.

While those who are already pregnant should stay covered outdoors to reduce the risk of bites.

Symptoms include flu-like aches, joint pain, rashes, itching, and inflammation of the eyes, headaches but not everyone has symptoms, according to government guidance on the virus.

It was first found in Africa in the 1940s, but was not a threat to humans.

There is currently no vaccine for the virus, but scientists at the University of Texas Medical Branch have visited Brazil to carry out research in order to find one.