JOHANNESBURG, South Africa (AP):

South Africa is sending more military doctors to its northern border to treat Zimbabwean cholera victims, underlining fears of a regional disease outbreak linked to Zimbabwe's collapse.

Cholera is easily prevented and cured, but Zimbabwe's medical and water-treatment systems have all but disappeared. The disaster has led to renewed calls - on Friday from the United States, among others - on longtime, increasingly autocratic Zimbabwean leader Robert Mugabe to step down.

Zimbabwe state media announced Thursday that a national health emergency had been declared. The United Nations estimates the cholera epidemic has killed at least 575, of at least 12,700 infected since August.

Aid agencies say it is likely many more Zimbabweans have sickened and died at home, officially unrecorded. Caroline Hooper-Box, spoke-swoman for the British charity Oxfam, said the death rate is as much as 10 per cent of those infected, while normally it is one per cent.

Contaminated food

Cholera is an infectious intestinal disease that is contracted by consu-ming contaminated food or water. Its symptoms include severe diarrhea.

South African government spoke-sman Themba Maseko said Friday that in addition to deploying more military health workers at the border, South Africa, the main regional power, was sending clean water and other aid into Zimbabwe.

South Africa also was dispatching a fact-finding team to Zimbabwe on Monday, Maseko said. Other huma-nitarian steps would be announced next week, he said, after the team returns and makes its report to the president and Cabinet ministers.

"We will continue to work with the World Health Organization's representatives and other donor organisations to provide assistance to medical facilities in Zimbabwe in order to manage and reduce the influx of Zimbabweans into South Africa and other neighbouring countries," Maseko said.

In Mozambique and Botswana, which also border Zimbabwe, health authorities were on alert, trying to determine the extent of the risk of cholera spreading from Zimbabwe. Cholera is common in many parts of southern Africa.

Zimbabwe's neighbours, led by South Africa, have been mediating in the political impasse, trying to get the increasingly autocratic Mugabe to implement a power-sharing agreement with his rivals. The main mediator, former South African President Thabo Mbeki, has kept to a policy of quiet diplomacy, but others in the region and beyond are losing patience with Mugabe.

Relinquish power

In an interview Thursday on the Dutch current affairs show Nova, Nobel peace laureate Desmond Tutu said African nations should use military force to depose Mugabe if he refuses to relinquish power.

Tutu, the retired Anglican archbishop of Cape Town, South Africa, said another option would be to threaten Mugabe with prosecution at the Hague-based International Criminal Court, although he did not say on what charges.

Mugabe "is destroying a wonderful country", said Tutu, who has long been among Mugabe's sharpest critics. "A country that used to be a bread basket ... has now become a basket case itself needing help".

United States Secretary of State Condoleezza Rice, speaking in the Danish capital Friday, said it was "well past time" for Mugabe to leave office.