South African President Jacob Zuma has begun a three-day visit to Zimbabwe, aiming to ease tensions within the fragile year-old unity government.
It is thought Mr Zuma will assess the country's readiness for an election, which could take place next year.
He was greeted at Harare's airport by both President Robert Mugabe and PM Morgan Tsvangirai, who agreed to share power after disputed elections in 2008.
They are at odds over key issues and have struggled to revive the economy.
The crisis in Zimbabwe has forced some three million people to flee across the border to South Africa, where they live as refugees.
'Year of anxiety'
When the Southern African Development Community (Sadc) appointed Mr Zuma as its chief mediator on Zimbabwe in 2009, Mr Mugabe's critics hoped he would take a tougher line on Zimbabwe's president than his predecessor, Thabo Mbeki.
But there has been little public evidence of a different approach to Mr Mbeki's policy of "quiet diplomacy".
According to South African officials, Mr Zuma is in Harare to "facilitate the removal of obstacles which hinder the full implementation of a power-sharing deal".
The BBC's Karen Allen in Johannesburg says it seems increasingly likely that Mr Zuma's focus will be ensuring that new electoral laws are in place in time for a possible vote next year.
Mr Mugabe recently said he would stand in the next election if his Zanu-PF party wanted him.
A spokesman for Mr Tsvangirai's Movement for Democratic Change (MDC), Nelson Chamisa, told the BBC's World Today programme that since the unity government was formed, "it has been a year of disappointment, a year of anxiety and suspense, a year of unfulfilled promises".
He said the only way of brining finality and closure would be to hold "free and fair elections under a new constitution".
But our correspondent says many Zimbabweans fear a repeat of the violence of the elections two years' ago.
The MDC has accused Mr Mugabe of breaking the power-sharing deal by unilaterally appointing a central bank governor and attorney general.
It also says its activists are still being harassed and beaten by Zanu-PF militants and members of the security forces.
The trial of senior MDC politician Roy Bennett on terrorism charges has further divided the two parties.
For his part, Mr Mugabe has angrily accused Mr Tsvangirai of not doing enough to have Western sanctions lifted.
On a recent visit to the UK, Mr Zuma also tried unsuccessfully to have those sanctions eased. He said he disagreed with the view that more pressure was required.
BBC News
