US President Donald Trump's cuts to HIV/AIDS programmes will further derail an already faltering plan to end the disease as a public health threat by 2030, United Nations Programme on HIV and AIDS (UNAIDS) Executive Director Winnie Byanyima said on Friday.

With 1.3-million new infections in 2023, according to the latest data, the world was already "off track," Byanyima told journalists in South Africa, a country with the world's largest number of people living with HIV, at 8-million.

"Less funding means we will get more and more off-track," she said in the main city of Johannesburg, after meeting President Cyril Ramaphosa to discuss Africa's HIV/AIDS strategy in light of the U S President slashing billions of dollars in foreign aid in February.

"We don't know yet what that impact will be, but impact there will be: ... already you see in several countries a drop in the number of people going to clinics," Byanyima said.

Before the cuts, prevention programmes had brought down new infections, she said, but they were "not coming down fast enough to reach our target of 2023."

Now, with the shuttering of community prevention clinics across Africa, infections would surely rise, though it wasn't clear yet by how much, she said.

The administration's decision to axe swaths of US foreign aid has disrupted the supply of life-saving HIV treatments, with some countries facing potentially running out. In South Africa, about a fifth of whose HIV budget was US -funded, testing and monitoring of HIV patients is already falling.

Byanyima said even poor, indebted countries were managing to plug funding gaps, but called on other rich nations to step in.

"We're saying to the donors: this is one of the diseases ... without a cure, without a vaccine, yet we're seeing progress," she said. "If you've got a good success story, why drop it ... before you end it?"