The programme aims to hand back land to blacks who were forcibly removed from their ancestral homes under apartheid, or offer them financial compensation.
This is part of efforts to correct skewed land ownership created under white minority rule.
President Thabo Mbeki's government wants 30% of farmland in black hands by 2014 but the transfer process has been slow, with only around four percent of land transferred so far.
Lulu Xingwana, the agriculture and land affairs minister, said wrangling with whites over price was one of the main reasons for the low turnover.
She said: "We are now going to negotiate six months - no more, no less.
"Indeed, we don't have time to be talking and talking for 10 years ... because already our people have been waiting.
"At least now we have ... expropriation. Therefore we will no longer waste time negotiating with people who are not committed to transformation."
South Africa has been quick to dismiss comparisons with neighbouring Zimbabwe - where a similar campaign was frequently marked by violence - and vowed to take a more orderly approach to addressing its apartheid and British colonial legacy.
Mbeki wants 30% of farmland in
black hands by 2014
Xingwana's department has already identified several properties that will be taken over if it cannot reach agreement over price with the owners, but it is the first time the government has set a time limit on such talks.
Officials stressed this would only be used as a last resort and farmers have the right to appeal against the decision in court.
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.
So far, 89% of the nearly 80,000 claims that were lodged by the December 1998 cut-off date have been settled.
The government has set a 2008 deadline to finish the process.