He said he had directed Secretary of State Mike Pompeo to look into it.
"The expropriation of land without compensation, our position is that would risk sending South Africa down the wrong path", State Department spokeswoman Heather Nauert said on Thursday.
South African President Cyril Ramaphosa. Thomson Reuters
Much of the farmland in South Africa is owned by the country's white minority, also known as Afrikaners, descended from Dutch settlers. No land has been "seized" since the reform plans were announced, the ANC said.
Last month, South Africa said it would go ahead with plans to amend the constitution, allowing land to be expropriated without compensation. Around 10% of land in white ownership has been transferred to black owners in this way, the BBC said.
He said this in light of Statistics South Africa (Stats SA) announcing in July that the official unemployment rate in the country had increased by 0.5% to 27.2% in the second quarter of 2018 from the first quarter. "There is black genocide in the U.S.; black people are killed every day".
A program of land redistribution is required to heal the historical "festering wound" of land dispossession and enable transformation and development, Ramaphosa told MPs in Parliament.
Commenting on the process that would be followed to implement land reform, Mabuza said, "everything we do will be done within the confines of the Constitution and within the existing legislative framework".
Agriculture accounts for less than 3 percent of national output but employs about 850,000 people, or 5 percent of South African's workforce.
MSNBC's Eddie Glaude was appalled by President Trump's decision to wade into South Africa's politics, and said the president was unnecessarily stoking racial tensions.
The State Department did not immediately respond to a request for comment about whether it has begun studying the issue. South Africa currently gets to export many products to the USA without duties, under the African Growth & Opportunity Act (AGOA), and some now fear that this access may be curtailed.
While violence against white farmers has declined in recent years, many far-right figures, including the conspiracy theorist Alex Jones, believe that white South Africans are in danger of being "wiped out".
Washington's charge d'affaires in South Africa, Jessye Lapenn, was summoned by the ministry of foreign affairs over the tweet, people familiar with the matter told Reuters.
Ramaphosa has said his land reform programme will be carefully carried out, to avoid what happened in Zimbabwe when former President Robert Mugabe's farm seizures sent the economy into a tailspin. "Trump's tweet on the land issue has been negative for the rand but we have to wait and see what comes out from that".
In this photo from October 30, 2017, people place white crosses, representing farmers killed in the country, at a ceremony at the Vorrtrekker Monument in Pretoria, South Africa.