A wider range of birth defects has been linked to the virus, said Dr Anne Schuchat of the US Centers for Disease Control and Prevention.

"Most of what we've learned is not reassuring," said Dr Schuchat at White House briefing on Monday.

"Everything we know about this virus seems to be scarier than we initially thought."

And the mosquitoes that carry the virus could travel to more US states than previously thought, she said.

The current Zika outbreak began almost a year ago in Brazil.

It has been linked to thousands of birth defects in the Americas.

Earlier this year, US President Obama asked the US Congress for $1.8bn (£1.25bn) in emergency funding to combat the virus.

In the meantime it has been using money totalling $589m left over from the Ebola virus fund.

(BBC News)