South Africa's foreign affairs ministry called in US Ambassador Leo Brent Bozell III on Tuesday for a third time over what it described as "undiplomatic conduct," a senior government official told Reuters, but did not elaborate on what the conduct was.

"We can confirm that we demarched Ambassador Bozell. The third demarche since he started his duties nine months ago," a senior official at South Africa's Department of International Relations and Cooperation (DIRCO) told Reuters late on Tuesday.

A demarche is a formal diplomatic protest from one government to another.

The US embassy did not immediately respond to a request for comment. The State Department acknowledged the receipt of questions from Reuters and said it would respond later.

Relations between the two countries have been strained by US President Donald Trump's repeated false claims that white South Africans face systemic persecution.

White South Africans are, on average, wealthier and more likely to be employed than Black South Africans. The government policies criticised by Trump are intended to address inequalities that persist from the apartheid era.

Bozell, a conservative activist and writer who arrived in Pretoria as Trump's envoy in February, was first called in within weeks of taking up the post over remarks that DIRCO said undermined South Africa's judiciary and historical context.

In recent interviews with local media, the ambassador has adopted a firmer and impatient tone, saying that Pretoria had not adequately addressed Washington's concerns and that further pressure could follow.

Two weeks ago, the US unveiled visa restrictions on some South Africans.

South Africa has maintained a willingness to engage in talks, but said that its sovereignty must be respected and that it will not compromise on domestic priorities like addressing historical injustices.

"The issues related to the bilateral relations between the two countries are a subject of ongoing discussions. There is commitment from both sides to resolve the outstanding matters," the DIRCO official said.