Zimbabwe's rival parties are to meet in South Africa for talks aimed at ending deadlock over a power-sharing deal.
Former South African leader Thabo Mbeki is to host the meeting between the ruling Zanu-PF and the opposition MDC.
The two sides have failed to agree on control of key ministries, but are under pressure to move forward amid a growing humanitarian crisis.
A cholera epidemic is sweeping Zimbabwe and South African officials say refugees are arriving with the disease.
Caroline Mahasela, mayor of Musina, a town 12km from the border with Zimbabwe, said about 70 cases had been reported so far. "People are scared, but they are trying to assist," she told the BBC.
At least 300 people have died from cholera in Zimbabwe in recent weeks and more than 6,000 cases have been reported.
On Monday former US President Jimmy Carter described the situation there as "much worse than anything we had ever imagined".
Speaking in Johannesburg after being refused entry to Zimbabwe, he said that the country's basic structure had "broken down", with provision of food, healthcare and education all serious problems.
'Basket of issues'
In recent days, South Africa - the regional power house, which is brokering the talks - has increased pressure on the two sides to reach an agreement.
Last week, it withheld $28m of aid until a representative government is formed.
On Monday, the leader of South Africa's governing ANC, Jacob Zuma, called on the two sides to implement the power-sharing deal "for the sake of Zimbabweans".
"It is now an urgent matter, people are dying," he said.
President Robert Mugabe's Zanu-PF and the Movement for Democratic Change (MDC), led by Morgan Tsvangirai, agreed to form a power-sharing government in September in the wake of disputed presidential elections.
The talks in South Africa will focus on a draft constitutional amendment intended to provide legal backing for the deal.
Once approved, it would enable Mr Tsvangirai to be sworn in as prime minister.
But the BBC's Peter Biles in Johannesburg says there are still other issues standing in the way of forming a new unity government.
The two sides have still not agreed on who will control the Ministry of Home Affairs, which has responsibility for the police.
The MDC said it wanted to discuss a "basket of issues" at the meeting, while the mood has soured since September.
"We have quite a mammoth task, particularly considering the insincerity, the inflexibility and the arrogance on the part of Zanu-PF," MDC spokesman Nelson Chamisa told the BBC's Network Africa programme.