The two sides in Zimbabwe's political crisis have asked for help from the regional grouping, Sadc, after talks on a power-sharing cabinet broke down.
After a fourth day of negotiations in Harare, opposition leader Morgan Tsvangirai said he and President Mugabe had failed to agree on key ministries.
Mr Mugabe said Friday's talks had gone "very well in the wrong direction".
But Thabo Mbeki, the former South African president who is mediating the talks said he was optimistic.
He said the outstanding issues were "capable of solving themselves quite easily", adding that Sadc officials would discuss Zimbabwe at a meeting in Swaziland on Monday.
The US meanwhile said it would consider further sanctions against Mr Mugabe and his government if the deal collapsed.
The BBC's Jonah Fisher in neighbouring South Africa says the month-old power-sharing agreement between President Mugabe's Zanu-PF and the two Movement for Democratic Change (MDC) factions now teeters on the brink of total collaps
Mr Tsvangirai said the parties had "failed to agree on the allocation of ministerial positions" in a new government of national unity.
"We believe that for an inclusive government to work, the principles of equitable sharing of power... should be embraced. It appears we are far apart on this principle," he told reporters.
The MDC leader then said both parties had agreed to call upon the Southern African Development Community (Sadc) and the African Union "to use their collective wisdom to help unlock the deadlock".
"We are not walking away from this. We hope that with the spirit of co-operation, there will be finality," Mr Tsvangirai added.
Mr Mugabe said the talks had gone "very well in the wrong direction", adding that a full statement would be made on Saturday.
The leader of a smaller MDC faction, Arthur Mutambara, who was also present at the talks, said he "saw partisanship and bankrupt ambition disrupting the discourse".
The latest power-sharing talks began on Tuesday as the parliament held its first working session under opposition control since disputed elections earlier this year. MPs heckled each other at the opening.
President Mugabe had reportedly told the MDC he had allocated the main ministries, including defence, home, foreign affairs, and justice, to Zanu-PF.
Mr Tsvangirai later told Mr Mbeki that 10 cabinet positions needed to be revisited at the negotiations, officials said.
As well as finance, the MDC also insisted it should have home affairs - and control of the police - if Zanu-PF had defence.
The MDC accused the security services of taking part in violent attacks on its supporters before June's presidential run-off election.
Reports on Friday suggested that Mr Mugabe might have agreed to let the MDC take the key post of finance minister, but that he would not give it home affairs.
Finance is one of the most crucial posts, as Zimbabweans hope the power-sharing deal will lead to action to tackle the economic crisis. Donors have promised to help finance a recovery plan but they are unlikely to release funds if a Zanu-PF minister is in charge.
At 231,000,000%, Zimbabwe has the world's highest rate of annual inflation, while some two million people need food aid. Aid agencies warn that figure could double within the next few months.
According to the original power-sharing deal agreed last month, 15 ministries are to be allocated to Zanu-PF, 13 to the MDC, and three to a smaller MDC faction led by Mr Mutambara.