The former UN secretary-general, ex-US President Jimmy Carter and rights advocate Graca Machel had planned to assess the country's needs. They are members of The Elders group, formed by former South African President Nelson Mandela to foster peace and tackle world conflicts.

Zimbabweans are suffering from disease and hunger while a political crisis over a power-sharing government occupies its politicians. A current cholera outbreak has killed nearly 300 people in Zimbabwe, the United Nations said.

But the three were told on Friday night by former South African President Thabo Mbeki, who is mediating the political crisis, that efforts to secure travel visas for the two-day trip had failed.

"We are very disappointed that the government of Zimbabwe would not permit us to come in, would not cooperate," former US President Carter said in Johannesburg.

It was the first time that Carter, the 2002 Nobel Peace laureate, had been denied permission to carry out a mission in any country.

Machel, a rights advocate for women and children who is married to Mandela, said she was denied a visa to visit Zimbabwe in July when she had planned to lead a women's delegation. Annan said no official reason had been given for the refusal.

Zimbabwe's state-run Herald newspaper reported last week that the group had been asked to "come at a later date" to accommodate the crop planting season. It quoted an unnamed source as saying the group was seen as antagonistic toward Zimbabwe's government.

Zimbabwe's main opposition leader Morgan Tsvangirai met the group at a hotel in Johannesburg, saying he was disappointed they could not meet under "better circumstances." He said, Zimbabwe's humanitarian crisis had reached "catastrophic levels."

Tsvangirai accuses President Robert Mugabe – who has been in power since Zimbabwe's 1980 independence from Britain – of trying to hold on to powerful cabinet posts despite agreeing to power-sharing talks.

The political impasse has left the country without leadership as its economy collapses, with deadly consequences. Lack of cash to maintain water and sewer systems, for example, has led to the cholera outbreak.

"It seems obvious to me that the leaders of the government are immune to reaching out for help for their own people," Carter said.

Zimbabweans face daily shortages of food, fuel and other basic goods. In the countryside, failed harvests mean that starving villagers are forced to scavenge.

As the country suffers from the world's worst inflation, health care has collapsed. Hospitals unable to afford drugs, equipment or staff salaries have been forced to shut down.

There is growing regional concern about Zimbabwe's crisis, as millions have left for neighbouring countries in search of jobs and security. "Any crisis that creates millions of refugees is regional, and everyone should be interested in resolving it," Annan said.