The United States has begun a phased withdrawal of HIV/AIDS funding for South Africa, a move that has prompted warnings from UNAIDS that hard-won gains against the epidemic could be at risk in the country with the world's largest HIV-positive population.

In a statement, the U.S. State Department said it had decided to initiate a gradual drawdown of PEPFAR support in South Africa, arguing that the programme was never intended to be permanent and that South Africa, as a middle-income country, is capable of funding its own health programmes.

The move comes against the backdrop of a broader review of U.S. foreign assistance under President Donald Trump's administration and follows months of tension between Washington and Pretoria over diplomatic and policy issues.

Reacting to the decision, UNAIDS Executive Director Winnie Byanyima said she was concerned about the implications for vulnerable communities that continue to rely on services supported by the programme.

"I'm sad about that. Taking it away is taking away life-saving support from the most vulnerable people. So, that is sad. And I would ask the United States to reconsider their position,"she said.