The US Centres for Disease Control and Prevention put its emergency operations centre on the highest level alert on Monday in response to the Zika virus.

The CDC decision came the same day the White House said it would ask Congress for 1.8 billion dollars in emergency funding to combat the mosquito-borne virus.

The CDC said its move was in response to the need for "accelerated preparedness" by experts in anticipation of Zika virus transmissions by mosquitoes in the continental United States.

The White House said its request for money would go toward expanding mosquito control programmes, speeding up research into a vaccine, enhancing diagnostic capabilities and helping Zika-affected countries in their fight to reduce transmissions, the White House said.

President Barack Obama said the Zika virus shouldn't cause people to panic.

"The good news is, this is not like Ebola. People don't die of Zika. A lot of people who get it don't even know they have it," said in an interview with CBS News.

Obama noted that the relationship between Zika and microcephaly, a condition in which babies are born with abnormally small heads, has not been proved conclusively.

The World Health Organization (WHO) has declared the Zika virus a public health emergency of international concern and said that up to 4 million people could be infected within the next 12 months in the Americas.

There have been 50 confirmed Zika cases in the US since December, the CDC has said. All were associated with travel to affected countries.

As the weather warms and mosquito populations return, there is the possibility that some areas of the US could see isolated cases, particularly in areas like southern Florida and Texas that have experienced other mosquito-borne illnesses, like dengue and chikungunya, CDC Principal Deputy Director Anne Schuchat said.

"We're optimistic that we won't have large-scale local transmission, but we're not betting on it and we want to do everything we can to make sure that we don't have wide-spread transmission here," she told reporters.

A dozen research groups were currently working on Zika vaccines, but all were still in the early stages, the WHO said in Geneva. The "availability of licensed products could take a few years," it said.

The top US infectious disease researcher said human trials of a vaccine could begin as soon as this summer.

Dr Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, said work on a vaccine is already under way, but cautions any vaccine might not be widely available for several years.

Because there are vaccines for similar types of viruses, Fauci said development should move fairly quickly.

It should not take much time to determine if a vaccine is safe and effective and that an accelerated approval process for an immunization against the disease might be possible, he said.

In addition, several studies on prophylactic medicines to protect against the virus are being carried out, the WHO said.

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