South Africa's ruling party says the health of Zimbabwe's aging President Robert Mugabe could affect talks on a new Zimbabwean constitution.
The African National Congress says in its weekly newsletter that negotiators are "concerned" about Zimbabwe's succession law should Mugabe "die or retire" before a new constitution is adopted.
Questions have been raised in recent months about the 87-year-old leader's health. Mugabe has traveled to Singapore at least five times since December.
Various media reports say the Zimbabwean leader is being treated for prostate cancer. A spokesman has denied the reports but said Mugabe did have an eye cataract removed during a trip in February.
Mugabe has ruled Zimbabwe since it won independence from Britain in 1980.
His ZANU-PF party is currently part of a contentious unity government with the longtime opposition Movement for Democratic Change. South African leaders helped broker the political agreement that led to the government.
The ANC says meditators suggest there is "serious progress" by Zimbabwe's political parties toward agreement on a roadmap to new elections in the country.
Some members of President Mugabe's ZANU-PF party are demanding elections be held this year.
ZANU-PF's coalition partner, the Movement for Democratic Change, says elections cannot be held until a new constitution is in place.