Media playback is unsupported on your device Media caption A new testing kit is being developed to identify infections quickly

US President Barack Obama has called for urgent action against the Zika virus, which has been linked to babies being born with underdeveloped brains.

He said research to develop vaccines and treatments must be speeded up.

The World Health Organization says the virus is likely to spread across nearly all of the Americas.

It has already been found in 21 countries in the Caribbean, North and South America. Symptoms include mild fever, conjunctivitis and headache.

Medics are most worried about the damage the virus can potentially cause to babies in the womb.

Brazil has recorded a huge spike in cases of microcephaly - the condition where babies are born with unusually small heads - and a number of Central and South American nations have asked women to delay pregnancy.

There have been 3,893 reported cases of microcephaly in Brazil since October compared with the previous annual average of just 160 cases.

Elsewhere:

A Danish tourist returning home from Latin America has been diagnosed with the virus. Earlier cases in Europe include Germany, Britain and Sweden

Health officials in the US states of Virginia and Arkansas say two residents who travelled outside America have tested positive for the virus, the AP news agency reported

Latin America's largest airline, Grupo LATAM, has announced it is waiving cancellation or flight-change fees for pregnant women wanting to cancel flights to affected countries

More on the Zika virus:

Image copyright Getty Images Image caption Babies born with abnormally small heads may face lifelong difficulties

Zika: What you need to know

Zika virus 'to spread across Americas'

Mothers' fears amid outbreak

'The worst day of my life'

The alarming threat of Zika virus

Virus 'may spread'

"The president emphasised the need to accelerate research efforts to make available better diagnostic tests, to develop vaccines and therapeutics, and to ensure that all Americans have information about the Zika virus," the White House said in a statement.

Media playback is unsupported on your device Media caption Viviane Lima created a WhatsApp support group to help answer questions from parents worrying about Zika virus

Mr Obama was briefed on the potential spread of the virus by senior officials on Tuesday, with one recent study suggesting that the virus may affect regions where 60% of the US population lives.

His spokesman said it was imperative for all Americans to have information about the virus and measures they can take to protect themselves.

US National Institutes of Health Director Dr Francis Collins cited a Lancet study earlier this month in which researchers warned the virus could spread in warmer months along areas of the east and west coasts of the US and much of the Midwest, where about 200 million people live.

The study also shows that nearly 23 million more people live in humid parts of the US where mosquitoes carrying the virus can survive all year round.

"It is now critically important to confirm, through careful epidemiological and animal studies, whether or not a causal link exists between Zika virus infections in pregnant women and microcephaly in their newborn babies," Dr Collins said.

Media playback is unsupported on your device Media caption Doctors say they are witnessing a sense of panic among patients, as Wyre Davies reports

Brazil's government on Tuesday said it would deploy 220,000 soldiers in its fight against the virus. They will go from home to home handing out leaflets on how to avoid its spread.

The announcement came after Health Minister Marcelo Castro said Brazil was "losing badly" in its fight against the virus.

What is Zika virus?

Image copyright AP