The failing health of Zimbabwe's President Robert Mugabe could jeopardise efforts to resolve his nation's political crisis, South Africa's ruling party said.
Mr Mugabe, 87, has travelled to Singapore five times since December to undergo medical treatment there.
The African National Congress, which serves as the chief mediator on Zimbabwe, said in an official newsletter that its mediators reported concerns that "should Mugabe die or retire" before constitutional reforms are complete, rivalry over his succession could delay elections meant to end the nation's troubled two-year coalition.
Mr Mugabe, who has ruled since independence from Britain in 1980, has said despite his age, he is fit to govern.
He called for elections this year, but regional mediators say that would be too early for free and fair polling.
The ANC said progress was made in recent negotiations over outstanding disputes between Mr Mugabe's party and the former opposition party, prime minister Morgan Tsvangirai's Movement for Democratic Change.
A power sharing deal between the two parties was brokered by the Southern African Development Community after disputed, violent elections in 2008.
But the timing for the adoption of a new constitution ahead of elections has remained an obstacle, the ANC said.
A referendum on constitutional reform is slated for September. Mr Tsvangirai insists fresh polls cannot be held before May 2012 at the earliest.
It was not clear if a regional summit on Zimbabwe will go ahead as planned on Friday in Namibia's capital, Windhoek.
The South African foreign ministry said its officials were waiting for the date to be confirmed.
The Windhoek meeting was not on the routine weekly schedule for South African President Jacob Zuma who heads regional mediation efforts. But Mr Mugabe's office said it received an invitation to go to Windhoek on Friday.
At the last regional summit in Zambia in March, Mr Mugabe and his party received a stern rebuke over the slow pace of reforms in Zimbabwe and continuing political violence.
A frail Mr Mugabe was transported around that summit venue in an electric golf cart.
The state media loyal to Mr Mugabe quoted George Charamba, Mr Mugabe's spokesman, as saying that Mr Tsvangirai misled the regional bloc by alleging Mr Mugabe was incapacitated and unable to hold office and that a "silent coup" had taken place, leaving generals and security chiefs in charge of the country.
Mr Tsvangirai and Western nations intended to turn the regional grouping against Zimbabwe and divide its leaders, Mr Charamba said.
"Our objective ... is to ensure that the correct and accurate situation in Zimbabwe is communicated" to regional leaders, he said.