MIAMI, United States (AFP) — Four cases of Zika virus in Florida are likely the first to be transmitted locally by mosquitoes in the United States, officials said Friday, marking a new phase in the fast-growing pandemic.

Until now, more than 1,600 cases of Zika – which can cause birth defects – have been recorded in the mainland United States but most were brought in by people who had become infected while traveling, with a smaller number transmitted by sexual contact.

In the past two weeks, investigators have determined “a high likelihood exists” that four suspected non-travel cases in Miami-Dade and Broward County “are the result of local transmission,” said the Florida Department of Health in a statement.

“At this time, the department believes that active transmission of the Zika virus is occurring in one small area in Miami-Dade County, just north of downtown.”

The US Centers for Disease Control confirmed the “cases are likely the first known occurrence of local mosquito-borne Zika virus transmission in the continental United States.”

CDC chief Tom Frieden said officials have no immediate plans to limit travel to the area.

“All the evidence we have seen indicates that this is mosquito-borne transmission that occurred several weeks ago in several blocks in Miami,” Frieden said in a statement.

“We continue to recommend that everyone in areas where Aedes aegypti mosquitoes are present — and especially pregnant women — take steps to avoid mosquito bites.”

Governor Rick Scott told a news conference that one of the cases involved a woman, and the other three men.

“They are all active Zika cases, and have not exhibited symptoms to be admitted to the hospital,” he said.

No tests on mosquitoes in the small area of southern Florida where the cases are located have come back positive for the virus, officials said.

– Birth defects –

Zika is spread via mosquitoes and by sexual contact. Pregnant women who are infected face a higher risk of bearing an infant with microcephaly, a birth defect that causes an abnormally small head.

Florida has already seen almost 400 cases of Zika, all involving people who were infected while traveling to parts of the world where the virus is circulating.

The United States has documented 1,657 cases of travel-related Zika in the past year, including 433 involving pregnant women, said Anthony Fauci, head of the National Institute for Allergy and Infectious Diseases.

“This is not unexpected,” said Fauci during a talk Friday at the Bipartisan Policy Center in Washington.

“I am almost certain that we going to see more.”

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

The virus has spread quickly throughout Central and South America and theCaribbean. The World Health Organization says 64 countries have reported mosquito-borne Zika since 2015.

The US territory of Puerto Rico has now diagnosed a total of 5,582 people, including 672 pregnant women, according to a separate CDC report out Friday.

“Puerto Rico is in the midst of a Zika epidemic,” said Lyle Peterson, incident manager for CDC’s Zika Response.

“This could lead to hundreds of infants being born with microcephaly or other birth defects in the coming year. We must do all we can to protect pregnant women from Zika and to prepare to care for infants born with microcephaly.”

Brazil has seen the highest number of birth defects linked to Zika, with 1,749 cases of microcephaly or central nervous system malformations potentially associated with Zika virus infection, according to the WHO’s latest report in July 28.

The news of local Zika transmission in Florida sparked a flurry of new calls for federal funding for the response.

“This is the news we’ve been dreading,” said Edward McCabe, senior vice president and chief medical officer of the March of Dimes.

“It’s only a matter of time before babies are born with microcephaly, a severe brain defect, due to local transmission of Zika in the continental US,” he added, calling for Congress to come to an agreement on President Barack Obama’s request for Zika funding.

In February, the White House requested $1.9 billion for Zika but lawmakers adjourned for the summer recess earlier this month without agreeing on legislation to fund the response.

“We urge (Congress) to approve this emergency funding necessary to control the impending crisis, which is now a direct threat to the health of the American people,” said Doctors for America executive director Alice Chen.