The situation in Zimbabwe may soon "implode" as a cholera outbreak spreads and basic services collapse, South African leaders and a group of international statesmen warned yesterday.

On the eve of talks in South Africa between Robert Mugabe's Zanu-PF party and opposition rivals, South African leaders sharply upgraded their crisis assessment and warned of Zimbabwe's imminent collapse if urgent action was not taken.

About 6,000 people have contracted cholera in recent weeks, according to the UN, and almost 300 have died. A chronic shortage of medicine has sent hundreds of people south to seek treatment in South Africa.

"Unless this root cause of the political absence of a legitimate government is solved, the situation will get worse and may implode and collapse ... It is now an urgent matter, because people are dying," said South Africa's caretaker president, Kgalema Motlanthe.

Three eminent statesmen representing a group called the Elders, which was established last year to tackle international issues, said Zimbabwe was in a far worse position than previously thought.

The team - the former UN secretary general Kofi Annan, former US president Jimmy Carter and Graça Machel, the rights activist and wife of Nelson Mandela - had planned to visit Zimbabwe last weekend to assess the humanitarian situation but were refused visas by the government. Instead they remained in South Africa talking to aid groups, refugees and civil society leaders.

Carter told reporters yesterday: "The entire basic structure ... is broken down. These are all indications that the crisis in Zimbabwe is much greater, much worse than we ever could have imagined."

The country has been locked in political stalemate since presidential elections in March when Morgan Tsvangirai, the leader of Zimbabwe's opposition Movement for Democratic Change, defeated Mugabe in the first round of voting. Tsvangirai pulled out of the June run-off however due to violence and intimidation of his supporters, allowing Mugabe to claim victory.

Attempts to agree a power sharing arrangement have foundered as Mugabe insisted on keeping most of the ministries for his Zanu-PF party.

In the meantime, the economy has disintegrated and the health system is close to breakdown. Four big hospitals, including two in Harare, have effectively closed their doors to new patients owing to a shortage of basic supplies and running water, the Elders said. Even women needing caesarean sections are being turned away.

Machel said it was clear that the state could no longer offer basic services and was failing its people.

"We have a sense that either the leadership doesn't have a clear picture of how deep the suffering is of their own people, or they don't care," she said.

The African National Congress leader, Jacob Zuma, who is likely to become South Africa's president next year, expressed concern about the cholera outbreak and said the ANC would send a delegation to Zimbabwe to assess the situation.

"We are dealing with a situation that is affecting the lives of people," he said.

South Africa has already threatened to withhold £18m in food aid to Zimbabwe unless a political solution is found.

Zuma and Motlanthe tried to increase the pressure on Mugabe and Tsvangirai to implement a power-sharing agreement and address the growing humanitarian crisis.

But unless Mugabe makes concessions, Tsvangirai will be left in a difficult position, forced to choose between accepting a weak position in government and holding out, leaving the country's dire socioeconomic situation to worsen.

Annan said that leaders of the Southern African Development Community had not yet put enough pressure on Zimbabwe to find a political solution.

"I think it is clear that SADC should have done more," he said after meeting Zuma and Motlanthe yesterday.

The Elders said they were shocked by the stories they had heard coming out of Zimbabwe.

"It is not just the extent of Zimbabwe's humanitarian crisis, but the speed of deterioration in the past few weeks that is most worrying," said Annan. "The scale, depth and urgency of the situation are under-reported."

The cholera epidemic has been caused by the collapse in the water and sanitation infrastructure. Cases have been reported in nine of the country's 10 provinces. Fatality rates are well above the international emergency rate of 1% due to a lack of drugs and medical assistance.