A unity government, delayed over the distribution of cabinet posts, is seen as the best chance of preventing total economic collapse in once prosperous Zimbabwe, where prices double every day and more than 2,000 people have died in a cholera epidemic.
On Sunday, however, both sides appeared entrenched in their positions.
President Robert Mugabe said the opposition MDC should accept the deal signed in September or end talks over its implementation, state media reported.
The MDC’s executive committee said it would not join a unity government until all its demands, including the equitable distribution of cabinet jobs with Mugabe’s ZANU-PF, were met.
The MDC called for "finality" in the dialogue, "either in success or in failure, because Zimbabweans cannot continue to be arrested by an inconclusive process", a statement said.
South African President Kgalema Motlanthe, who chairs the Southern African Development Community (SADC), will lead the delegation, including Mozambique’s President Armando Guebuza and former South African President Thabo Mbeki, in Monday’s talks in Zimbabwe.
Commentators doubt the latest round will break the deadlock.
"Since they signed the deal, the parties are further apart and there is a strong likelihood of the total collapse of the political agreement," political commentator and Mugabe critic John Makumbe told Reuters.
"It is very likely that ZANU-PF will go it alone and form a minority government. It is also very likely that without an agreement, the humanitarian crisis will take a sharp turn for the worse."
Monday’s meetings follow Mugabe hinting there would be no further talks with the opposition after the latest round.
"This is the occasion when it’s either they accept or it’s a break. After all this is an interim agreement. If they have any issues they deem outstanding, they can raise them after they come into the inclusive government," Mugabe was quoted as saying in the Sunday Mail.
Opposition leader Morgan Tsvangirai arrived back in the country on Saturday, the first time he has been in Zimbabwe since November. He has spent much of the time in neighbouring Botswana.
Tsvangirai won a presidential election last March but by too few votes for an outright victory. He pulled out of the subsequent run-off, citing violence against MDC supporters.
As he arrived back, he told reporters he hoped Monday’s meetings would find a "lasting solution", but added: "The MDC will not be bulldozed into an agreement that doesn’t reflect the will of the people of this country."