NAIROBI, Kenya — The South African government has given white farmers six months to sell their land or be forced to leave under the country’s post-apartheid land restitution program.

The ruling African National Congress promised to return 30% of white-owned farms to black South Africans when it came to power in 1994.

But so far, only 4% of that land has been transferred. Most of the white farmers have agreed to sell their land or pay claimants compensation, but tortuous bureaucracy means that land or funds have not yet been transferred.

The deadline — the first time that the South African government has imposed a time limit — is aimed at the 8,107 white farmers who so far have refused the prices they are offered or have demanded proof from those claiming to be the original owners. The government says it must speed up the process to meet its December 2008 deadline to complete redistribution.

“We will no longer waste time negotiating with people who refuse to see the transformation of our country,” the agriculture and land affairs minister, Lulu Xingwana, said. She announced the policy shift over the weekend in Limpopo province in the country’s agricultural heartland.

“From now on, we will only negotiate for six months, and if all fails, expropriation would take place.”

The program was set up to give land seized by the white minority during British colonization or under apartheid back to black people who can prove that they have a genuine claim to specific plots.

Nationwide, 79,696 claim forms were lodged before the government’s cut-off deadline in 1998. In Limpopo alone, landless blacks have applied for the return of 99.8% of the province’s commercial farmland. Many applicants lack documents linking them to particular parcels of land. In some cases, family trees proving ancestral claims have had to be painstakingly drawn up.

Mrs. Xingwana’s department has already identified several properties that will be taken over if owners refuse to agree on a sale price.

Land claims are an especially emotive part of the post-apartheid reforms, as they often bring white families who have lived on the land for generations up against blacks whose historical ties to the land run even deeper.

But the government has been quick to dismiss comparisons between its land policies and those in neighboring Zimbabwe, where forced and often violent redistribution of white-owned farms has crippled the country’s agricultural economy.

Mrs. Xingwana stressed that South Africa would seize land only as a last resort if all other attempts at a “willing buyer, willing seller” arrangement failed. Landowners could also appeal against the decisions in the courts, she said. Critics of the process disagree not with the principle of giving land back but with what will happen to the highly profitable farms after they are acquired.

Many black farmers lack the experience or the capital to run large commercial farms. The Agriculture Ministry has established a 10-year handover program, whereby white and black farmers can run farms as a joint venture until the new owners have acquired the skills to maintain their profitability.