South African President Thabo Mbeki has said his country will have to live with an influx of illegal immigrants from neighboring Zimbabwe
Up to three million are thought to have fled to South Africa, amid a worsening economic and political crisis.
There is high unemployment, and fuel and food shortages across Zimbabwe.
Addressing parliament, Mr Mbeki said it was not possible to put "a Great Wall of China" between the two countries and stop people walking across the border.
The South African authorities regularly deport Zimbabwean illegal immigrants, but many make repeated attempts to enter South Africa, in the hope of finding refuge and employment.
They often find work in the farms of northern South Africa, where some complain they are exploited.
The annual rate of inflation in Zimbabwe has soared to 3,731.9% - by far the highest rate in the world, official figures show.
President Mbeki has been appointed by Southern African leaders to mediate between Zimbabwe's governing Zanu-PF party, and the opposition Movement for Democratic Change, in the hope that free and fair elections can be held next year.
Mr Mbeki said on Thursday the discussions were going "very well" but he gave no details.
It is understood however that the government and the opposition in Zimbabwe have yet to hold a face-to-face meeting.
Mr Mbeki has always preferred "quiet diplomacy" to public criticism of President Robert Mugabe's government.
But recently, senior ANC official Toxyo Sexwale said he feared that the Zimbabwe government was not listening and the "volume may have to be turned up".
Waiting for Thabo Mbeki
Zimbabwe short of bread and flour