US officials have ruled out a vaccine to protect against Zika in the next few years as concerns continue to mount about the spread of the virus.
The World Health Organisation (WHO) announced on Thursday that Zika was "spreading explosively" throughout the Americas and "the level of alarm is extremely high".
The WHO has set up an International Health Regulations Emergency Committee to examine Zika and will meet on Monday to decide whether it constitutes a global emergency. The last time a global emergency was declared was for the Ebola virus.
Zika, which has spread to 23 countries, has been linked to thousands of babies being born with underdeveloped brains in Brazil.
Colombia has also seen a rise in the number of patients diagnosed with a rare neurological disorder which can cause paralysis.
The US has said it has two potential candidates for a vaccine for the Zika virus. While clinical trials may be able to begin before the end of this year, there will not be a widely available vaccine for several years.
Dr Anthony Fauci, director of the National Institute for Allergy and Infectious Diseases, said one of the vaccines was based on work done on the West Nile virus.
He told a news conference that officials were already talking to drug companies about taking a vaccine forward.
In Colombia, health minister Alejandro Gaviria has reported a "substantial increase" in the number of people with Zika reported to have Guillain-Barre syndrome, which is rare in the UK.
It is a serious condition of the peripheral nervous system and most people (around 60 per cent) develop it after having a viral or bacterial infection.
Experts believe the infection may trigger the immune system to attack nerve roots and peripheral nerves.
The WHO predicts three to four million people will be infected with Zika in the Americas this year.
In a briefing to the WHO's executive board on Thursday, WHO director-general Margaret Chan said the organisation was "deeply concerned".
"Arrival of the virus in some places has been associated with a steep increase in the birth of babies with abnormally small heads and in cases of Guillain-Barre syndrome," she said.
"A causal relationship between Zika virus infection and birth malformations and neurological syndromes has not yet been established, but 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."
Women in Britain have been warned by Public Health England (PHE) to consider avoiding travel to areas where Zika is active.
PHE and the National Travel Health Network and Centre (NaTHNaC) issued updated advice for pregnant women on Wednesday.
Dr Dipti Patel, director at NaTHNaC, said: "We strongly advise all travellers to avoid mosquito bites and urge pregnant women to consider avoiding travel to areas reporting active Zika transmission.
"If travel to these areas is unavoidable, or they live in areas where Zika virus transmission is occurring, they should take scrupulous insect bite avoidance measures both during daytime and night-time hours."
Since the start of the outbreak in 2015, five British travellers have been diagnosed with the Zika virus.
Symptoms of infection may include fever, joint pain, itching, rash, conjunctivitis or red eyes, headache, muscle pain and eye pain.
Brazilian experts at the Oswaldo Cruz Foundation have said the virus – which was thought to be confined to the Aedes aegypti mosquito in the tropics – may have already crossed over to the culex mosquito, a much more common type.
But WHO sought to play down these fears, saying there was no reason to suspect culex mosquitoes are transmitting the virus.
The Aedes aegypti – also known as the yellow fever mosquito – does not occur in the UK.
Professor Michael Bonsall, from the University of Oxford, said it was "very unlikely" Aedes aegypti mosquitoes could live in the UK "because they are a tropical and subtropical beast".
"There are around 30 other species of mosquitoes in the UK of which about 1/2 are in the Aedes genus [group] but are different species," he said.
