WASHINGTON — The Obama administration on Thursday said it was shifting $81 million away from biomedical research and antipoverty and health care programs to pay for the development of a Zika vaccine, resorting to extraordinary measures because Congress has failed to approve new funding to combat the virus.

Sylvia Mathews Burwell, the secretary of health and human services, told members of Congress in a letter that without the diverted funds, the National Institutes of Health and the Biomedical Advanced Research and Development Authority would run out of money to confront the mosquito-borne illness by the end of the month. That would force the development of a vaccine to stop at a critical time, as locally acquired cases of Zika infection increase in Miami.

As of last week, 7,350 cases of Zika had been reported in the United States, most in Puerto Rico, according to the Centers for Disease Control and Prevention. Ms. Burwell said that 15 infants had been born with Zika-related birth defects. The virus can cause abnormal brain development and other serious defects in children born to infected mothers.

The local spread of the illness in the continental United States, with the first cases reported late last month, has raised the political stakes surrounding the federal government’s response. Hillary Clinton on Tuesday made a campaign stop in Wynwood, the Miami neighborhood that has had a rash of locally transmitted Zika cases, and pressed Congress to return from its five-week break to approve emergency funding to fight the virus.