The source close to the negotiations, which have been described as a last-ditch effort to save the power-sharing agreement, told Reuters they were "almost certain to collapse".
The assessment came just after regional leaders started talks with Mugabe and opposition parties in the latest push for implementation of September’s deal.
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.
South African President Kgalema Motlanthe, Mozambique leader Armando Guebuza and Thabo Mbeki, regional mediator and South Africa’s former president, arrived at a Harare hotel for the talks shortly before noon (1000 GMT) on Monday.
Mugabe, Movement for Democratic Change (MDC) leader Morgan Tsvangirai and the head of breakaway MDC faction Arthur Mutambara were due to try to thrash out an agreement to save the pact.
"Now is the time to find common ground among Zimbabweans. Now is the time for flexibility, compromise and pragmatism," Mutambara told reporters at the venue.
"As the opposition we share and agree with the demands made by MDC-T. Those demands must be addressed. We hope Robert Mugabe and ZANU-PF respond positively to those demands."
The party called for "finality" in the dialogue, "either in success or in failure, because Zimbabweans cannot continue to be arrested by an inconclusive process".
Mugabe said the MDC should accept the deal signed in September or end talks over its implementation, state media reported.
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.
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."