US President Barack Obama called on Congress Saturday to step up funding to combat the Zika virus, warning that delay is putting more Americans at risk.

Obama's latest appeal, in his weekly radio address, came the day after the US authorities expressed deepening worry about the spread of the mosquito-borne virus, urging that all donated blood be tested for Zika.

The Congress has denied past administration requests for Zika funding, instead redirecting funds that had been earmarked to fight Ebola, cancer and other diseases.

‘That's not a sustainable solution,’ Obama said. The delay for more funds ‘puts more Americans at risk.’

Congress ‘should treat Zika like the threat that it is’ and ‘fully fund our Zika response,’ he added. ‘A fraction of the funding won't get the job done. You can't solve a fraction of a disease.’

Zika infections in pregnant women can cause a severe birth defect known as microcephaly, in which babies develop abnormally small skulls and brains.

More than 2,500 people in the United States have been diagnosed with Zika, along with more than 9,000 in Puerto Rico and other US territories, according to the Centers for Disease Control and Prevention.

Most of those cases were brought in by people infected while traveling abroad.

There are 584 pregnant women on the US mainland with lab evidence of Zika infection, and 812 in the US territories.

Florida in July announced its first cases of locally transmitted Zika, with 42 infections.

- Revised guidelines -

The Food and Drug Administration revised its guidelines for blood donations on Friday, recommending that all donated blood be tested for the Zika virus.

Its previous guideline issued in February recommended active screening of donated blood only in ‘areas with active Zika virus transmission.’

Since there is ‘still much uncertainty regarding the nature and extent of Zika virus transmission,’ the recommendation for testing all donated blood ‘will help ensure that safe blood is available’ for everyone, said Peter Marks, director of the Food and Drug Administration's Center for Biologics Evaluation and Research.

Stricter national safeguards are needed as evidence has emerged that Zika can be transmitted sexually, and those infected often show no symptoms, the FDA said.

Donated blood is already being tested in Puerto Rico and Florida, where at least one unit of blood was found to contain the Zika virus and was intercepted, Marks told reporters on a conference call.

Expanded blood supply testing ‘will be in effect until the risk of transfusion transmission of Zika virus is reduced,’ the FDA said.

Zika is primarily spread by the bite of an Aedes aegypti mosquito, but it can also be transmitted sexually.

The US authorities on Friday announced the first known case of a man who had Zika but did not know because he showed no symptoms -- and subsequently infected his female partner during unprotected sex.

Four out of five people who get Zika do not show any of the common symptoms, which may include fever, rashes, joint pain and red eyes.

‘As new scientific and epidemiological information regarding Zika virus has become available, it's clear that additional precautionary measures are necessary,’ the FDA's acting chief scientist Luciana Borio said.

The World Health Organization says 53 countries around the world have reported Zika outbreaks since 2015.