South Africa's president said on Thursday the US had signalled it might change its mind and participate in the G20 summit in Johannesburg after a boycott by the Trump administration, but the White House dismissed the report as "fake news."

Cyril Ramaphosa was speaking at a joint news conference with European Commission President Ursula von der Leyen and European Council President António Costa.

"We have received notice from the United States, a notice which we are still in discussions with them over, about a change of mind, about participating in one shape or form or other in the summit," he said.

"This comes in the days before the summit. And so therefore we need to engage in those types of discussions to see how practical it is and what it finally really means."

US President Donald Trump's administration had said it will not attend the first G20 summit in Africa, alleging that the host country, previously ruled by its white minority apartheid system until 1994, discriminates against white people.

On Thursday, a White House official said an envoy would attend a ceremony for the official handover of the G20 presidency from South Africa to the US but there was no question of Washington participating.

"This is fake news. The chargé d'affaires in Pretoria will attend the handover ceremony as a formality, but the United States is not joining G20 discussions," they added.

But South African presidential spokesperson Vincent Magwenya said "the president will not hand over to a chargé d'affaires."

Trump has rejected South Africa's agenda for the November 22-23 summit of promoting solidarity and helping developing nations adapt to worse weather disasters, transition to clean energy and cut their excessive debt costs.

Ramaphosa said last week of the handover of the G20 presidency to the United States: "I don't want to hand over to an empty chair, but the empty chair will be there."