South African President Cyril Ramaphosa dismissed on Sunday US President Donald Trump's threat to exclude Pretoria from next year's G20 summit, reaffirming South Africa's status as a founding member of the group.

Washington boycotted the Group of 20 leaders' summit held under South Africa's presidency in Johannesburg on November 22-23, with Trump repeating allegations, widely discredited, that the host country's Black-majority government persecutes its white minority.

Trump said last Wednesday that South Africa would not be invited to the summit to be held in Florida next year because it had refused to hand over the G20 presidency to a senior representative of its embassy who was at the closing ceremony. Pretoria says it handed over the rotating presidency to a US embassy official.

"South Africa is and will remain a full, active and constructive member of the G20," Ramaphosa said in a state of the nation address on Sunday.

He also described as "blatant misinformation" Trump's repeated claims that South Africa was committing "genocide against Afrikaners" - descendants of Dutch settlers - and confiscating land from white citizens.

Ramaphosa noted that despite the diplomatic rift, US businesses and civil society groups engaged actively in G20-related events in Johannesburg in November.

"We value those constructive ties and will continue to work within the G20 framework," he said, signalling Pretoria's intention to maintain dialogue.