Русская версияSouth African President Thabo Mbeki is trying to secure a deal between his Zimbabwean counterpart, Robert Mugabe, and the opposition's Morgan Tsvangirai.
Mr Mbeki is hoping to a power-sharing government may be agreed on.
Mr Mugabe and Mr Tsvangirai are due to meet in a Harare hotel after more than a week of talks between their parties, reports say.
Reports in some South African papers say a deal is close, and that a final agreement could be reached shortly.
One widely touted solution is that Mr Mugabe may become ceremonial president with Movement for Democratic Change leader Morgan Tsvangirai being made executive prime minister.
But there has been no official comment on these reports, apart from statements from all sides that the talks have been progressing well.
Mugabe's role
Mr Mbeki has been mediating negotiations between the two sides in neighbouring South Africa.
Zimbabwean government spokesman George Charamba described Mr Mbeki's visit to Harare as an "important milestone".
He said the South African president was "going to meet the principals, basically to update them on the progress so far and to consult on how to take the dialogue forward".
Mr Mugabe won a run-off in June after Mr Tsvangirai pulled out of the race alleging violence against his backers.
South African mediators say that talks are aimed at creating some form of coalition but there is disagreement over who would lead a unity government and over Mr Mugabe's exact role.
For the two men to achieve a power-sharing deal will be no easy matter, as they share nothing but a mutual loathing, says BBC Africa analyst, Martin Plaut.
Make or break
Assuming a deal can be achieved, the next question is how the senior figures in the country's security apparatus - the military and police - can be accommodated, our analyst adds.
There were strong suggestions a deal was blocked earlier this year because their concerns had not been addressed.
This time round it is proposed that there would be two deputy prime ministers:
- one - to be occupied by the ruling Zanu-PF, would take the defence portfolio
- the other, for the MDC, would take the police
If this is acceptable to all sides, our analyst suggests, then other issues are seen as less important.
But Sunday's negotiations are seen as make or break, with diplomats warning that until everything is agreed, nothing is agreed, he adds.
Summit threat
Mr Mbeki is under pressure to produce a solid outcome ahead of a mid-August summit of the Southern African Development Community (Sadc).
Hundreds of thousands of Zimbabweans have fled the country's worsening political and economic situation, many crossing over the borders into neighbouring states of South Africa, Zambia and Botswana.
Botswana's foreign minister suggested on Friday that Mr Mugabe should be barred from the Sadc summit.
(BBC)
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