JOHANNESBURG (Reuters) - Zimbabwe's opposition MDC will not sign an accord leading to talks on ending the political crisis until mediator Thabo Mbeki, South Africa's president, has addressed some concerns, a party spokesman said on Sunday.

Sources in the Movement for Democratic Change said on Saturday that MDC leader Morgan Tsvangirai might sign the document as early as Monday so that talks could begin on ending an impasse with President Robert Mugabe's ruling ZANU-PF.

Mugabe easily won re-election on June 27 in a second-round poll from which Tsvangirai pulled out, citing violence against his supporters by ZANU-PF militias.

On Sunday Tsvangirai's spokesman, George Sibotshiwe, said the MDC leader would not sign until Mbeki, criticised for his failure to help end the stand-off, ironed out concerns with parts of the memorandum, which sets out guidelines on substantive negotiations.

"I think in principle the decision is to sign the document. We are committed to the dialogue process," Sibotshiwe told Reuters.

"Our executive and council have already gone through the document and have raised their concerns with the facilitator ... the onus is on the facilitator to ensure that those things are sorted out in order for the signing to happen within the required time."

Asked whether a signing was likely on Monday, Sibotshiwe replied "I cannot answer that. (Mukoni) Ratshitanga, the spokesperson for President Mbeki, is the only person who can respond to that."

Ratshitanga said he was not aware of any plans for Mbeki to travel to Zimbabwe "any time soon" and declined to comment on what concerns the MDC had raised.

ECONOMIC CRISIS

The MDC has refused to recognise Mugabe's overwhelming victory in the controversial June 27 vote.

The stalemate has dented hopes of halting an economic crisis in Zimbabwe widely blamed on the policies of Mugabe, in power since independence from Britain in 1980.

The meltdown has shown itself in record inflation of well above 2 million percent, chronic shortages of basic food and other commodities, and extremely high unemployment.

Mugabe blames the crisis on economic sabotage by Western enemies he says have supported the MDC as revenge for the government's seizure of white-owned farms for blacks.

Critics say Mbeki's mediation efforts have made no progress because his soft diplomacy is slanted in Mugabe's favour.

Tsvangirai won Zimbabwe's first-round presidential vote in March. Official figures showed he did not get the absolute majority needed to avoid a second-round election, but the MDC insists its leader won outright the first time.

On Sunday Kenyan Prime Minister Raila Odinga said he expected the MDC and ZANU-PF to sign an agreement this week for talks to take place in South Africa which he hoped would lead to a safe exit from power for Mugabe.

"Robert Mugabe is an embarrassment to the African continent," Odinga told BBC television. "He lost an election and refused to move on."