Mugabe and Movement for Democratic Change (MDC) leader Morgan Tsvangirai failed to reach a deal on forming a unity government at a meeting on Monday which was attended by regional leaders.
They agreed to try again to break the deadlock at a summit in either Botswana or South Africa next week held by the regional Southern African Development Community (SADC). But the MDC said hopes for success were slim.
"We are doubtful whether SADC will be able to deal with this issue," MDC spokesman Nelson Chamisa told Reuters.
A unity government is seen as the best chance of preventing total collapse in once prosperous Zimbabwe, where prices double every day and more than 2,000 people have died in a cholera epidemic.
But a September power-sharing agreement has stalled amid fighting over who should control key ministries and regional leaders have failed to secure a compromise, despite international calls for stronger action.
Mugabe’s government accuses Tsvangirai of frustrating efforts to form a new government but the MDC says Mugabe’s ZANU-PF is trying to relegate it to being a junior partner and has vowed not to join any new administration if its demands are not met.
These include control of powerful ministries such as finance, home afairs and information.
"We cannot afford to be in an inclusive government where we don’t feel included," Chamisa said.
MDC secretary general Tendai Biti said there was "no question of us boycotting. We will be attending (the SADC summit)".
But he was "not hopeful".
"You can have a million extraordinary summits but as long as no one (in the SADC leadership) has the courage to look Mugabe in the face and tell him, old man, logic has to prevail, it will be meaningless."
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.