South Africa's ex-President Thabo Mbeki is holding talks with Zimbabwe's political leaders to salvage last month's power-sharing deal.
He is meeting Zimbabwe's President Robert Mugabe and opposition leader Morgan Tsvangirai in a Harare hotel.
Negotiations stalled after President Robert Mugabe allocated key ministries to his Zanu-PF party at the weekend.
In parliament, MPs are due to discuss changing the constitution, so Mr Tsvangirai can become prime minister.
Before the talks began, Mr Mbeki's spokesman said he was confident he would be able to rescue the deal, which he brokered just before stepping down as president in September.
"We are convinced that we should be able in the end, no matter how long it takes, to reach a conclusion," said Mukoni Ratshitanga.
This is Mr Mbeki's first direct intervention since he resigned. Correspondents say it is not clear if he will still wield the same clout in the negotiations.
Mr Tsvangirai has threatened to pull out of the deal after a government list published on Saturday gave the main ministries, including defence, home, foreign affairs, and justice, to Zanu-PF.
He wants all cabinet positions to be revisited in discussions with Mr Mbeki, but Zanu-PF says only one ministry - finance - is up for discussion.
"As far as we are concerned, the only contention is the Ministry of Finance," Zanu-PF negotiator Patrick Chinamasa told the state-run Herald newspaper.
According to the deal - which allocates 14 ministries to Zanu-PF, 13 to Mr Tsvangirai's Movement for Democratic Change (MDC) and three to a smaller MDC faction - only Zanu-PF has a ministerial seat vacant.
Mr Mbeki arrived in the Zimbabwean capital, Harare, on Monday night.
Local journalist Brian Hungwe says he looked at ease as he arrived in Harare, smiling and waving to people.
But political analysts say Mr Mbeki has little room to manoeuvre given the stance of both parties.
Richard Dowden, director of the UK's Royal African Society, says Mr Mugabe has "made a mockery of the agreement".
"I think what the MDC will do is appeal directly to [ANC leader Jacob] Zuma where they know they have someone more sympathetic to them than Mbeki," Mr Dowden told the BBC's Today programme.
"[The South African government] might begin to use the sort of instruments people have been urging Mbeki to use for years - just simple public disapproval of what Mugabe is doing might have an affect."
On Sunday, Mr Tsvangirai said that if Zanu-PF wanted the defence ministry, the MDC must have home affairs, which controls the police.
"We are still placing our faith in the efforts of the mediator, and that Zanu-PF has to be persuaded that it has to share and not grab power," MDC spokesman Nelson Chamisa said, Reuters news agency reports.
Before Mr Mbeki's arrival, Mr Mugabe swore in two vice-presidents.
The European Union condemned Mr Mugabe's "unilateral decision" to form a new government and ministers hinted that they could extend sanctions against Mr Mugabe and his close allies.
On Thursday, it was announced that Zimbabwe's annual inflation rate had soared to 231,000,000% - by far the highest in the world.
The UN says two million people are in need of food aid, and that the figure will rise to 5.1 million - or 45% of the population - by early 2009.