Much of the United States government’s funding for AIDS treatment and research is funneled through the President’s Emergency Plan for AIDS Relief, or Pepfar, which was established in 2004 by President George W. Bush in an effort to save Africa from an epidemic that threatened to kill much of the population of entire countries, like Botswana and Namibia.

President Barack Obama expanded Pepfar, and combined with the Global Fund and other international efforts, the spending is widely credited with arresting the AIDS epidemic. About 37 million people worldwide are infected with H.I.V., including nearly two million children. About one million people died of AIDS in 2015, and two million were newly infected that year.

Pepfar funds anti-AIDS activities in more than 60 countries. But in the briefing on Tuesday, Mr. Sastry said the Trump administration planned to ensure that the United States was “focusing our efforts in the 12 high-burden countries to achieve epidemic control.”

He did not name those 12 countries, but in past years, the program focused much of its work on a dozen African countries, as well as Haiti, Vietnam and Guyana.

The Trump administration has also proposed eliminating $524 million in funding for contraceptives and other family planning efforts that mostly benefit women in developing nations.

Melinda Gates, co-founder of the Bill & Melinda Gates Foundation, said in a statement posted on her Facebook page that the proposed family planning cuts “would lead to more unintended pregnancies, more maternal deaths.”

“This budget threatens to trap millions more families in a cycle of poverty,” she said.

It is unclear how many lives could be lost as a direct result of the budget cuts, but the Global Fund estimates that every $100 million invested saves about 133,000 lives. An amfAR calculation found a similar effect, suggesting that the administration’s proposed cuts to AIDS programs alone could cost more than one million lives and orphan more than 300,000 children.

“All of these programs have multiplier effects beyond just those immediately served by them,” said J. Stephen Morrison, who directs global health work at the Center for Strategic and International Studies. “For the first time ever, after 15 years of steady growth, we’re going to see a radical regression that will have huge effects.”