"The proposal was being presented to Sadc heads of state and obviously we wait to hear from them. I think the Sadc heads of state are engaging with the leadership of [Mugabe's] Zanu-PF and seeing if we can find common ground," Sibotshiwe said.
A South African official close to the negotiations said remaining points of contention included whether Mugabe would retain the right to hire and fire ministers and how long a transitional government would remain in place.
According to a copy of a speech obtained by The Associated Press news agency, Tsvangirai told Sadc ministers: "We have agreed that Mr Mugabe will be president whilst I become prime minister.
"We envisage that the prime minister must chair the cabinet and be responsible for the formulation, execution and administration of government business, including appointing and dismissing his ministers."
Sibotshiwe declined to give specific details of how the power-sharing agreement would work, but appeared to dismiss claims that Mugabe would retain control of the military as commander-in-chief under the deal.
"I think the issue is that the army is a professional army, what we are concerned about is the abuse of that army, and if Tsvangirai is head of government then whoever is in cabinet as a minister of defence would be answerable to Morgan Tsvangirai," he said.
The MDC leader has also said he will not enter into any deal which would not see him having real power.
"It's better not to have a deal than to have a bad deal," Tsvangirai told The New York Times newspaper on the sidelines of the Sadc summit.
"Who is in charge of the cabinet?" he said. "To whom do all these ministers report? Can you dismiss them if they breach? It's fundamental."
Three days of discussions in Harare, the Zimbabwean capital, earlier this week between Mugabe, Tsvangirai and Arthur Mutambara, who leads a breakaway faction of the MDC, ended after Tsvangirai refused to agree to a proposed power-sharing deal.
But Thabo Mbeki, the South African president who is mandated to mediate an end to post-election turmoil in Zimbabwe, has said he remains confident that a resolution is still possible, raising hopes that an end to the impasse can be found at the summit.
The Zanu-PF also said that negotiations would continue at the two-day Sadc summit.
If a deal is reached it would see Tsvangirai working alongside the man he blamed for the widespread intimidation and violence cited as the reason he pulled out a presidential run-off vote on June 27.
Tsvangirai had come first out of four candidates in the first round of presidential voting in March, but failed to win by the margin necessary to avoid a run-off against second-place finisher Mugabe.
Mugabe was declared the winner after he stood unopposed, despite observers criticising the poll.