"They (the U.N. and AU) are the guarantors of the (power-sharing) agreement. If ZANU-PF continues to be intransigent, then those institutions should step in," MDC spokesman Nelson Chamisa said.
Supporters of Mugabe earlier accused the MDC of "bad faith" and of stalling on a deal to try to bring about U.N. mediation.
Chamisa said the MDC was ready to compromise, but "not to the point of betrayal", and would not settle for being the junior partner in a Mugabe-led government.
"Once they start to show they are willing to show sensitivity to the suffering of the people, I have no doubt we will be able to pull through," he said by telephone when asked about the prospects for a deal.
Mutambara, arriving earlier for the talks, said "flexibility and pragmatism" must force the opponents to end the impasse, which threatens the broader deal brokered by former South African president Thabo Mbeki a month ago.
"We share the pain and suffering of our people. Surely, this must force us to agree on this debate on cabinet and move on to the core business of running our country," he told reporters.
The power-sharing deal is seen as Zimbabwe’s best hope for rescuing an economy where fuel, food and foreign exchange are scarce, and inflation stands at 231 million percent. Reuters