Aid groups say Zimbabwe faces a major humanitarian crisis
Former US President Jimmy Carter has said the crisis in Zimbabwe is "far worse" than he had imagined.
Mr Carter is one of a group of world leaders, known as the Elders, who were refused entry visas for Zimbabwe to assess the humanitarian situation.
He was speaking in Johannesburg after talks with South Africa's leaders.
Zimbabwe's rivals are to meet on Tuesday in South Africa to salvage a power-sharing deal, South Africa's president has announced.
The head of the country's ruling African National Congress, Jacob Zuma, said the situation in Zimbabwe was beyond "wait and see". "We have got to act and act now," he said.
In a joint statement, the Elders - a group set up to tackle world conflicts - said the scale of the crisis "must be acknowledged and addressed by Zimbabwe's leaders".
They also urged neighbouring countries to act immediately to tackle the problems of cholera and refugees.
Speaking in Johannesburg on Monday, Mr Carter said: "The crisis in Zimbabwe is much greater, much worse than anything we had ever imagined."
The three members of the Elders group were refused entry to Zimbabwe
He described the government in Harare as unwilling to communicate and said President Robert Mugabe did not want to admit that there was a crisis, preferring to blame problems on what Mr Carter called "non-existent sanctions".
"There are no economic sanctions against Zimbabwe and [Mr Mugabe] does not want to admit that there is a need for assistance," he said.
The country's basic structure was "broken down", Mr Carter added, with provision of food, healthcare and education all serious problems.
He pointed to the cholera epidemic that has swept Zimbabwe, killing at least 300 people and affecting some 6,000.
All political parties should implement the power-sharing deal as a matter of urgency and work to form an inclusive government
Donors should give enough to meet emergency humanitarian needs, including $140m to feed people until April; humanitarian agencies must have free access
Region should set up medical task force to treat those affected by cholera and contain its spread
Government, helped by donors, should ensure emergency treatment of water sources
Government, helped by aid agencies, should encourage teachers back to school with adequate pay, housing and food
SADC region needs to acknowledge that the crisis in Zimbabwe is affecting its neighbours and should be addressed immediately
Normally the disease would be expected to kill 1% of patients, Mr Carter said, but the toll in Zimbabwe was much higher because huge inflation meant hospitals could not afford basic drugs.
He added that as the crops had failed this year, the next harvesting opportunity would be April 2010.
Fellow Elder and former UN Secretary General Kofi Annan added that there was a $140m shortfall between the aid provided by the international community and what was required.
The international aid community would need to find $550m next year, he said.
He also said that the regional group SADC (Southern African Development Community) could have - and should have - done more to tackle the situation.
Kofi Annan's fears for Zimbabwe
Aid groups say Zimbabwe is facing a major humanitarian crisis, with nearly half the population needing food aid by early next year.
Nelson Mandela's wife, Graca Machel, the third member of the group and a human rights activist, said: "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."
The three have said the sole aim of their trip on behalf of the Elders was to help people in Zimbabwe, and that they had no intention of becoming involved in any political negotiations.
The Elders' comments came shortly after Mr Zuma told reporters: "The situation has gone [beyond] where we could say 'wait and see'.
"We are pleading for the leadership [of the ruling party and opposition] for the sake of the people to find a solution that would help them move forward."
It is now an urgent matter, people are dying
Jacob Zuma ANC leader
He described as an "unfortunate act" Zimbabwe's decision to refuse visas to Mr Carter, Mrs Machel and Mr Annan.
Mr Zuma said the ANC would be sending a delegation to Zimbabwe to push for a political solution to the crisis.
Zimbabwean President Robert Mugabe's Zanu-PF and the opposition Movement of Democratic Change (MDC) are in a power-sharing stand-off following disputed presidential elections earlier this year.
They have agreed to form a government of national unity but been unable to agree on who should fill key ministries.
"Let us find a way to implement the agreement for the sake of Zimbabweans," Mr Zuma said. "We cannot stay with the agreement without implementing it. It is now an urgent matter, people are dying."
These are some of his strongest words so far on the situation in Zimbabwe, the BBC's Peter Biles in Johannesburg says.
Last week, South Africa said it would withhold some $28m of aid until a representative government was formed.
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