Zimbabwe's Robert Mugabe came under pressure from South Africa to swear in his rival as prime minister even as his ruling party threatened new elections on Saturday over power-sharing disputes.

A draft constitutional amendment paving the way for a unity government was published in the government gazette Saturday, the first step toward bringing it to parliament for approval, Justice Minister Patrick Chinamasa told the state-run Herald newspaper.

South African President Kgalema Motlanthe welcomed the draft amendment, which creates the positions of prime minister and vice-prime minister, saying he expected the incumbents to be sworn in "with immediate effect".

Motlanthe said the gazetting of the constitutional amendment was a "major step towards the formation of an inclusive government in Zimbabwe".

The amendment creates the position of prime minister, earmarked for opposition Movement for Democratic Change leader Morgan Tsvangirai, but rival parties have still not agreed on key issues which could derail the process.

"In the event that the collaboration that we envisage is not forthcoming, then that will necessitate fresh harmonised elections at some point in time," Chinamasa said in the paper.

The MDC won control of parliament for the first time in March elections, but does not have enough seats to approve the amendment on its own.

Negotiators for Mugabe and Tsvangirai have agreed to a draft of the text, but MDC spokesman Nelson Chamisa warned that key issues surrounding the unity government remain unresolved.

"There are still some outstanding issues which need to be resolved before the bill goes through. These issues which are political in nature relate to issues of appointment of governors, ministers and others," Chamisa told AFP.

Tsvangirai won a first-round presidential vote in March, but pulled out of a runoff after a deadly campaign of violence, which he accused Mugabe's party of orchestrating.

Mugabe and Tsvangirai signed a unity accord three months ago, but so far have failed to agree on how to form a cabinet, leaving government in limbo as the nation confronts a cholera epidemic that has killed nearly 800 people.

UN estimates say the disease has already infected more than 16 000 people, and aid agencies warn the epidemic could last for months and cause 60 000 cases.

Mugabe this week sparked an uproar by claiming that the cholera epidemic had been conquered, even as the death toll mounts daily.

His information minister, Sikhanyiso Ndlovu went on to accuse Britain of causing the epidemic as part of a "genocidal" attack on the country.

The MDC derided the 84-year-old president as either "mischievous or genuinely out of touch with reality".

Mugabe has ruled his country since independence from Britain in 1980, when he had been welcomed as a liberator.

Now he is under growing international pressure to step down, with Britain, France and the United States loudly calling for him to leave power.

UN chief Ban Ki-moon told reporters Friday that he had met with Mugabe two weeks ago at a summit in Qatar and urged him to leave his legacy "in a positive way".

"Mugabe really should look for the future of his country and his own people, who have been suffering too much, too long from this political turmoil now coupled with very serious humanitarian tragedies," Ban said.

But Ban said the meeting did not go well, calling the dialogue "very difficult".

US Secretary of State, Condoleezza Rice, plans to hold talks at the UN headquarters in New York next week, a spokesman said.

"We're in discussions with members of the Security Council as to what the Security Council as a body might do," State Department spokesman Sean McCormack said Friday.

"And what we want to do is to start a process that will bring an end to the tragedy that is unfolding in Zimbabwe."

The United States is talking to South Africa, seen as having the most influence with Mugabe, as well as other countries about finding a solution, McCormack indicated.

A senior US official, speaking on condition of anonymity, proposed on Thursday that Zimbabwe's neighbours, particularly South Africa, close their borders with the country so that it yields to international demands.

AFP