But Zimbabwean opposition leaders said on Sunday the accord was in jeopardy after Robert Mugabe, the president, decided to award key posts to his ruling party.
Nelson Chamisa, the spokesman for the opposition Movement for Democratic Change (MDC), headed by Morgan Tsvangirai, made a passionate appeal to Mbeki and the 15-member regional Southern African Development Community (SADC) bloc to help resolve the latest crisis.
"Mr Mbeki, please help Zimbabwe. We need your help. We also need the help of and support of the SADC," Chamisa told SA FM radio.
In power since Zimbabwe's 1980 independence from Britain, Mugabe's decision, announced on Saturday, to award defence, home and justice ministries to his Zanu-PF ruling party means he would retain control of the army, police and other state security apparatus.
A government notice carried by the state-run Herald newspaper said on Saturday that the veteran leader had given his Zanu-PF party 14 ministries, including foreign affairs, justice, local government and the all-important state media.
"It kills the talks completely. This flies in the face of the dialogue and an attempt by the SADC to help us out of this crisis. Clearly, it is an act in bad faith"
Nelson Chamisa, spokesman for the opposition MDC
"It kills the talks completely," Chamisa said. "This flies in the face of the dialogue and an attempt by the SADC to help us out of this crisis. Clearly, it is an act in bad faith."
Chamisa said that the move was "arrogant, unilateral and unacceptable".
Edwin Mushoriwa, spokesman for the breakaway MDC faction led by Arthur Mutambara, which also signed the Mbeki-brokered deal to share power, said that Mugabe's announcement stemmed from "hallucination" on the part of ZANU-PF.
"That [ministerial] list is what they wish to happen. It was not agreed on. As far as we know there was no agreement on the allocation of cabinet posts and we are waiting for the mediator, Mr Mbeki, to come and resolve the impasse," he told the AFP news agency.
According to the Herald, Tsvangirai's MDC gets 13 posts, covering portfolios such as constitutional and parliamentary affairs, economic planning and investment promotion, labour and social welfare and sport, the arts and culture.
Mutambara's MDC would get three ministries.
Under the September 15 accord, Mugabe, 84, remains as president while Tsvangirai takes the new post of prime minister.
Saturday's shock announcement came hours after negotiators for the three main parties in the stand-off, including Zanu-PF, urged Mbeki to step in once again to resolve the deadlock over the allocation of the ministries.
Luckson Ruvire, a Zimbabwean student in Harare, said: "There is no power-sharing if we are to judge by the list. It is still Zanu-PF running the government because they have all the main ministries."