"There are outstanding issues such as the issue of governors, equity and allocation of key ministries which have to be addressed," said Movement for Democratic Change spokesperson Nelson Chamisa.
"Unless those issues are resolved, we cannot be invited to be passengers and be bystanders in a government we are supposed to be partners. No amount of propaganda against us would force to jump into this government," he said.
But the ruling Zanu-PF a new government could not be held to ransom by party leader Morgan Tsvangirai after the latest failed regional mediation effort.
"If they are not interested, I do not see why there cannot be a government. They will never hold this country to ransom," deputy information minister Bright Matonga was quoted as saying in the government mouthpiece The Herald on Wednesday.
Mugabe has said a new government would be put in place "as soon as possible", while his lead negotiator Patrick Chinamasa said Tsvangirai had been asked to submit names for ministers.
Their comments came after Tsvangirai rejected a proposal by regional leaders at the weekend to immediately form a unity government and share the disputed home affairs ministry with Mugabe, dashing hopes of a breakthrough.
Mugabe and Tsvangirai agreed to share power in September but have failed to break a deadlock on key cabinet posts which has sent Zimbabwe into further economic free-fall and stopped foreign donors from stepping in.
The political feuding has dashed hopes of ordinary Zimbabweans that their daily struggle for survival could ease.
With inflation running at more than 231 million percent, half of the population requires emergency food aid while a breakdown in basic services has led to deadly outbreaks of cholera in Harare.
Western nations have said they are ready to release hundreds of millions of dollars in aid, but not while Mugabe retains his grip on power.