MIAMI, United States (AFP) – US regulators Thursday called for a halt to blood donations in the Miami area as investigators probe four potential non-travel associated cases of the mosquito-borne Zika virus, which can cause birth defects.

If confirmed, the cases would mark the first time that mosquitoes carrying the virus are known to be present in the mainland United States.

The US territory of Puerto has already seen a surge in local transmission of Zika, which can spread by mosquitoes or sexual contact.

“In consideration of the possibility of an emerging local outbreak of Zika virus, and as a prudent measure to help assure the safety of blood and blood products, FDA is requesting that all blood establishments in Miami-Dade County and Broward County cease collecting blood immediately,” said a statement from the US Food and Drug Administration.

This freeze should remain in place until each individual unit of blood collected in the two counties can be tested “with an available investigational donor screening test for Zika virus RNA or until the blood establishments implement the use of an approved or investigational pathogen inactivation technology,” it said.

Two of the suspected non-travel cases are in Miami-Dade County, and two are in Broward County which is just to the north of Miami.

The FDA also said anyone who has traveled to Miami-Dade or Broward county in the past four weeks should be temporarily barred from donating blood.

“Additionally, FDA recommends that adjacent and nearby counties implement the precautions above to help maintain the safety of the blood supply as soon as possible,” said the federal agency.

Florida has already seen 381 cases of Zika, all involving people who were infected while traveling to parts of the world where the virus is circulating.

For Zika to become a homegrown virus in the mainland United States, a mosquito would have to bite a Zika-infected person and then bite another person, passing on the virus.

Health officials have warned of possible localized Zika outbreaks in the United States, particularly since the virus has spread quickly throughout Latin America and the Caribbean in the past two years.

If a pregnant woman is infected with Zika, it raises the risk of her bearing an infant with microcephaly, a permanent defect which results in children being born with unusually small heads.