President Thabo Mbeki (l) has been the lead mediator over Zimbabwe
Zimbabwe's opposition says talks with Robert Mugabe's party to solve the political crisis are "deadlocked".
MDC spokesman George Sibotshiwe told the AFP news agency that he was unable to give any further details.
Unnamed sources say an offer to make opposition leader Morgan Tvsangirai third vice-president was "insulting".
Talks began last week in South Africa after a rare meeting between Mr Mugabe and Mr Tsvangirai, who both claim victory in this year's elections.
The delicate negotiations are meant to be happening under a news blackout in a secret location near the South African capital Pretoria.
'Lack of sincerity'
The Associated Press news agency reported that Mr Mugabe's negotiators were to fly home to Zimbabwe on Monday, although it is unclear whether the talks are in recess or have broken down completely.
AP quoted a different official in Zimbabwe saying Justice Minister Patrick Chinamasa and Social Welfare Minister Nicholas Goche - Mr Mugabe's negotiators - might be going home to consult the president about their mandate.
The same official said Mr Tsvangirai had left Zimbabwe on Monday and was travelling to Pretoria, to consult his own negotiators.
"We cannot discuss the main issues, we can only say that they are in a deadlock and that the parties will consult with their principals," Mr Sibotshiwe said.
"If the sticking points are resolved then the talks will resume," he said.
A source quoted by AFP said the proposal to name Mr Tsvangirai third vice-president showed a "complete lack of sincerity and the need to really address the issues and problems Zimbabwe is facing".
Zimbabwe's current first and second vice-presidents are both high-ranking members of Mr Mugabe's Zanu-PF party.
A spokesman for President Mbeki told the BBC he had no knowledge of the talks breaking down.
The talks are supposed to result in some form of power-sharing arrangement, although the details are unclear.
The agreement to talk did not mention the central issue - whether Mr Mugabe would remain as president.
The agreement, hailed as historic when it was signed last Monday, said a final deal should be agreed within two weeks.
The South African government earlier criticised new sanctions imposed on Zimbabwe by America and the EU, saying they could obstruct the talks.
Mr Tsvangirai pushed Mr Mugabe into second place in the first round of voting on 29 March but he pulled out of a 27 June run-off election after a wave of deadly attacks against his supporters.
The Movement for Democratic Change (MDC) says that more than 120 of its supporters have been killed, some 5,000 abducted and 200,000 forced to flee their homes after being attacked by Zanu-PF militias and security agents.
Zanu-PF officials deny the charges.