Zimbabwe's opposition said Friday it had reached some "understanding" with President Robert Mugabe, despite earlier claims that a latest round of power-sharing talks had made no progress.

"There's been some shared understanding on the issue of the constitutional amendment which will set out the powers of the new prime minister," Movement for Democratic Change (MDC) spokesman Nelson Chamisa told AFP.

"However other outstanding issues are still to be resolved."

The constitutional amendment is critical to the forming of a unity government, which has run aground over the share of powers between Tsvangirai and Mugabe under a two-month-old deal.

The opposition's announcement comes two days after Tsvangirai said former South African president Thabo Mbeki should step down as the mediator in the political crisis and that fresh talks had made no headway.

"He does not appear to understand how desperate the problem in Zimbabwe is, and the solutions he proposes are too small," Tsvangirai said in a statement issued on the second day of the talks in Johannesburg.

Chamisa declined Friday to say if he was optimistic that a unity government would soon be formed, but said he believed the opposition could settle its differences with the ruling ZANU-PF.

"There are some issues which are remaining that should not take a lot of energy and time," he said.

"A lot depends on the sincerity of ZANU-PF."

The South African media printed Friday details of a scathing letter written by Mbeki to Tsvangirai in which he accused the MDC leader of stonewalling the power-sharing talks.

The 4000-word letter, printed in The Star newspaper, was written before the latest round of talks ended and slammed Tsvangirai's refusal to accept recommendations of regional leaders on forming a unity government, accusing him of seeking support from the West rather than from African neighbours.

Mbeki said Tsvangirai should "take responsibility for the future of Zimbabwe, rather than see its mission as being a militant critic of President Mugabe and ZANU-PF."

Mbeki took particular issue with Tsvangirai's remarks that southern African leaders "did not have the courage" to stand up to Mugabe at a summit in Johannesburg on 9 November.

Regional leaders and Mbeki's mediation efforts have come under increasing pressure to push the bickering Zimbabwean politicians to agreement, as the country faces a deadly cholera outbreak which has claimed 389 lives.

South Africa's new president Kgalema Motlanthe has withheld millions of dollars in aid until a new government is in place, and Botswana has urged the international community to tell Mugabe "you are on your own".

Zimbabwe's unity deal was hailed as a step toward hauling Zimbabwe out of political turmoil and economic ruin, but instead the nation has sunk deeper into crisis.

The explosion of cholera is the latest sign of the collapse of the country which was regarded as a post-colonial success story in the first two decades after independence from Britain in 1980.

Now Zimbabwe is now burdened by the world's highest rate of inflation — last put at 231 million percent.

Mugabe's government suffered another blow Friday, as a regional tribunal ruled that 78 white Zimbabweans can keep their farms because the government's land reform scheme discriminated against them.

Judge Luis Mondlane, president of the Southern African Development Community (SADC) tribunal based in Namibia, said that Zimbabwe had violated the treaty governing the 15-nation regional bloc by trying to seize the white-owned farms.

Sapa