Ideas of white racial superiority are a threat to South Africa's post-apartheid unity, its sovereignty and its diplomatic relations, President Cyril Ramaphosa said on Monday, calling for global efforts to debunk false stories about white persecution.

Ramaphosa was speaking at a conference of the co-ruling African National Congress (ANC), the party of Nelson Mandela that defeated racist white minority rule in South Africa three decades ago.

"Some in our society still adhere to notions of racial superiority and seek to maintain racial privilege," Ramaphosa said, adding that their views "conveniently align with wider notions of white supremacy and white victimhood fed by false claims of the persecution of white Afrikaners (descendants of mostly Dutch settlers) in our country." 

Two weeks ago, US President Donald Trump boycotted a Group of 20 summit in Johannesburg, alleging without evidence that South Africa mistreats its white minority, and that white farmers were "being killed and slaughtered, and their land and farms are being illegally confiscated".

The US administration was furious that the hosts went ahead and secured a G20 declaration anyway, and said South Africa would be excluded from the next one, to be held in the US.

Though widely debunked, the notion that South Africa's white minority is suffering a "genocide" has been circulating in far-right and white supremacist chat rooms for years, where aggrieved white nationals have sought support from some US Republicans and the European right for their cause. 

In February, Trump gave it as a reason for cutting development aid to South Africa. 

"These false claims (have) ... real implications for our sovereignty, international relations and national security," Ramaphosa said. "A campaign ... needs to be launched not only in our country, but globally ... to address (them)."

The US is South Africa's third most important export market after the European Union and China. Trump's administration has imposed a 30% import tariff on its goods, resisting South African attempts to have it reduced.

"Our government is continuing to engage the United States in negotiations to reach a trade agreement that benefits both countries," Ramaphosa said. "At the same time, we have had to accelerate diversification of our export markets."