"Our understanding is that he (Tsvangirai) indicated to the AU guy that he is now ready to come on board, and that the MoU is likely to be signed on Monday," an official, who declined to be identified because of the sensitivity of the talks, said.
"The MDC wanted a direct AU involvement in the mediation, and we have that now . and unless something major crops up, the signing should happen on Monday or Tuesday," he added.
There was no immediate word from Mugabe, the ruling ZANU-PF or Ping, but Ping was expected to meet with the Zimbabwean leader for discussions later.
Mbeki has been mediating preliminary talks between ZANU-PF and the MDC to resolve the stand-off but critics say he has not made progress and has favoured Mugabe with his soft approach.
Tsvangirai won the first round presidential vote on March 29 but official figures showed he failed to get the absolute majority needed to avoid a second ballot. The MDC insists Tsvangirai won outright the first time.
On Saturday SADC Executive Secretary Tomaz Salomao said after a meeting of the regional grouping's foreign ministers that the inclusion of the AU and U.N. was meant to boost the Zimbabwe mediation efforts.
"They (AU and U.N. officials) are not part of the negotiation team but they will be based in Pretoria to support the efforts that we as a region are trying to undertake," Salomao told reporters in South Africa's port city of Durban.
"We are working hard, very hard . to ensure that by the time we meet at the summit, progress can be reported to the summit," Salomao added, referring to SADC's annual summit scheduled for early August.
Zimbabweans are suffering chronic shortages of meat, maize, fuel and other basic goods because of the collapse of the once prosperous economy, which the MDC and other critics blame on Mugabe's seizure of white-owned farms and other policies.
Central bank Governor Gideon Gono said on Wednesday inflation had surpassed 2.2 million percent, but some economists say it is actually much higher.
In a notice in the official Herald newspaper on Saturday, Gono said the Reserve Bank of Zimbabwe would introduce Z$100 billion special agro-cheques (notes) to help consumers forced to carry large wads of cash even for simple transactions.
The Zimbabwe dollar, officially pegged at 30,000 to the U.S. dollar before exchange rules were relaxed recently, now trades at about 800 million to the U.S. unit.
