"It is critical that we conclude this dialogue as a matter of urgency."
Morgan Tsvangirai, the leader of the MDC, is said to have agreed to become prime minister in a national unity government, although the exact balance of power remains unresolved.
An MDC official said mediation efforts would continue on Monday and Tsvangirai would begin a regional tour to consult with southern African leaders.
Focus on economy
Thirteen heads of state in the SADC had hoped the two sides would reach an agreement at the summit and move on with efforts to rescue Zimbabwe's shattered economy.
Negotiations began last month after Robert Mugabe was re-elected president unopposed in June, in a vote condemned around the world and boycotted by Tsvangirai because he said his supporters were being attacked.
A troika of SADC leaders called on Zimbabwe's parties to quickly reach a power-sharing agreement and said parliament may have to be convened as talks continue, said Thabo Mbeki, the South African president and chief mediator in the talks.
But the troika did not impose a deadline and it was not possible to say when a deal would be reached, said Mbeki, the new chairman of the 15-nation SADC.
Officials close to the talks have said the main sticking point is who would have control of the government.