"This is becoming a matter of extreme concern to us and we will be taking quite a hard stance to make sure that agreement is reached," cabinet spokesman Themba Maseko told reporters.
The 15-nation Southern African Development Community (SADC) is scheduled to hold an emergency summit in South Africa on Sunday to discuss the political stalemate in Zimbabwe.
A smaller SADC meeting held in Harare last month failed to end the deadlock.
President Robert Mugabe, opposition leader Morgan Tsvangirai and Arthur Mutambara, leader of a smaller breakaway opposition faction, agreed to share power on September 15, but talks have stalled over control of ministries.
Maseko added: "We believe that South Africa and the region cannot be held to ransom by three parties that are failing to reach agreement on the allocation of cabinet posts."
Establishing a unity government is seen as critical to reversing an economic meltdown in the southern African nation where inflation is officially put at 231 million percent. Even under government price controls, the cost of bread is doubling every week.
Zimbabweans are struggling to survive amid widespread shortages of meat, milk and other basic commodities as a result of the collapse of the agricultural sector. The country is dependent on food handouts and malnutrition is on the rise.
Tsvangirai, who would become prime minister under the power-sharing deal, has accused Mugabe’s ZANU-PF of trying to seize the lion’s share of important ministries to try to relegate the MDC to the role of junior partner.
Tsvangirai won a March presidential election but not with enough votes to avoid a run-off. He then pulled out of the second vote, citing violence against his supporters.
He left Zimbabwe on Wednesday to visit countries in the region ahead of the Sunday summit, travelling on a temporary travel document after the government failed to issue him with a passport, the MDC said.
Tsvangirai snubbed a planned SADC meeting in Swaziland last month after being denied a passport.