Johannesburg - South Africa on Monday heaped pressure on Zimbabwe's rival political leaders to form a unity government, with President Kgalema Motlanthe and the man tipped to soon replace him, Jacob Zuma, describing the situation in Zimbabwe as "very bad" and "very desperate."

Motlanthe, chairman of the 15-nation Southern African Development Community reiterated: "The parties must implement with immediate effect their own agreement which they signed on September 15."

He was speaking after meeting with former United Nations secretary general Kofi Annan, former US president Jimmy Carter and Mozambican social activist Graca Machel, who were barred from travelling to Zimbabwe at the weekend by President Robert Mugabe's regime.

The three, representing The Elders brains trust of leading activists and former world leaders founded by anti-apartheid icon Nelson Mandela in 2007, wanted to visit Zimbabwe to shine a light on the worsening humanitarian crisis in the country.

Close to 300 people have died of cholera and a further 6,000 have been infected with the diarrhoeal disease in recent weeks as health, sanitation and other public systems in the once model African economy collapse. The Elders had been due to meet health workers and non- governmental organizations struggling to feed over 3 million hungry people.

But Mugabe's government denied them visas, saying they had not consulted officials about their planned visit. The Elders, who met in South Africa with opposition leader Morgan Tsvangirai, Zimbabwean refugees and aid agencies working with Zimbabweans, say they had asked, in vain, for a meeting with Mugabe.

Speaking after talks with the three Elders, Motlanthe said Monday they had painted a "very depressing picture" of the situation in South Africa's neighbour to the north, which is increasingly being felt south of the border.

Four people have died of cholera in South Africa over the past 10 days as ailing Zimbabweans stream across the porous border in search of treatment.

Already struggling to contain resentment in poor communities at the influx of an estimated 2 million Zimbabwean refugees, South Africa, the regional powerbroker, is showing signs of losing patience.

Ruling African National Congress (ANC) leader Zuma, who is expected to replace Motlanthe as president after elections next year, called Mugabe's refusal to allow their visit, an "unfortunate act" and said the ANC would send envoys to meet with Mugabe's Zanu-PF and Tsvangirai's Movement for Democratic Change (MDC).

Despite agreeing in principle in September to govern together, the two parties have been unable to agree how to divvy up power.

The MDC is balking particular at Mugabe's insistence that Zanu-PF and the MDC share control of the police. The MDC wants full control of the police in return for allowing Zanu-PF to control the army.

Last week, South Africa announced it would follow the example of Western donors and withhold a substantial farm aid package until the dispute was resolved. (dpa)