Lobby group AfriForum has claimed its work in the United States led to President Donald Trump's tweet on land expropriation and murders on farms.

"We welcome it," said Ernst Roets. The group travelled to the US in May to lobby individual members of the US Senate and the House of Representatives.

"I think our lobbying has certainly had an impact because we have spoken with a lot of people who have had contact with President Trump and we have spoken with many think tanks, one of them for example the Cato Institute, which has taken a very strong stance shortly before this statement now by President Trump."

Trump tweeted that he had asked his secretary of state to look into land expropriation and the killing of farmers in South Africa.

"I have asked Secretary of State @SecPompeo to closely study the South Africa land and farm seizures and expropriations and the large scale killing of farmers. South African Government is now seizing land from white farmers," Trump tweeted.

This was after watching a discussion on the Tucker Carlson show on Fox News, where Carlson opened a debate with: "South Africa's government has begun seizing the property of some of its citizens based on its skin colour. Does our government care? No. They applaud it."

He said the state department had just sent them an "unbelievable statement" which "should make you mad".

He said President Cyril Ramaphosa had been "seizing land from its own citizens without compensation because they are the wrong skin colour".

"That is literally the definition of racism. Racism is what our elites say they dislike most.

"'Donald Trump is a racist' they say, but they pay no attention to this at all. In fact, Ramaphosa is one of Barack Obama's most favourite leaders in the world. And this means just a few weeks ago, Obama praised him. He praised the racist government of South Africa and Ramaphosa by name for quote, 'inspiring hope throughout the country'," Carlson said.

Roets said the lobby group had already received messages of thanks for its visit to members of the US Senate and House of Representatives in May.

He said he expected anger from the majority party, the African National Congress (ANC), but believed this would be "hypocritical" because the party's members had used the same international lobbying against the apartheid government.

"I hope that it would inject a degree of sanity into the debate that we have in South Africa at the moment, because it seems like those who are pushing for expropriation without compensation are doing so irrationally," he said.

Meanwhile, the Presidency wants clarity on the tweet.

"The Presidency has noted President Trump’s tweet and will take up the matter through diplomatic channels," said spokesperson Khusela Diko.

"To this end, the Presidency will request [the minister of the department of international relations and co-operation] to obtain a clarification from the US ambassador."

Ramaphosa met with representatives of agricultural group, AgriSA on Tuesday.

In Parliament on Wednesday, he criticised those "spreading lies and rumours" like AfriForum, who went "overseas saying the ANC is after a land grab".

Ramaphosa insisted, as he had done since the ANC adopted a resolution at its conference in December, that the process must be orderly.