Hundreds of thousands of people in the US territory of Puerto Rico could become infected with the Zika virus in the coming months, according to the director of America’s Centre for Disease Control.
Dr Tom Frieden says this could lead to “thousands” of brain-damaged babies.
Zika has now been reported in 31 countries and territories in the Americas, with Brazil the worst hit.
There have been about 100 cases of Zika reported in mainland US.
These were in travellers who had recently returned from Zika-hit countries.
The Aedes aegypti mosquito that spreads the virus can be found in about a dozen US states, so the mainland is bracing itself for locally transmitted infections very soon.
Southern states such as Florida and Texas are particularly vulnerable.
But Dr Frieden – who is advising the president on this emergency – says Puerto Rico is the biggest concern for the US.