President Cyril Ramaphosa described the death of Chad President Idriss Déby Itno in military violence as "disturbing news", and expressed concern on behalf of the South African government about developments in that country.
He said Déby died "following injuries he sustained while leading his military forces to repel a rebel attack".
In a statement issued hours after Déby's death on Tuesday, Ramaphosa said he received the news with sadness and extended condolences to the government and people of Chad.Ramaphosa said the involvement of armed groups in the violence was a concern.
He added that an immediate cessation of violence was necessary.
We condemn in the strongest terms the violence that has claimed so many lives, including that of President Déby. We call for calm and a cessation of fighting.
Déby died a day after provisional election results that were published showed that he was elected for a sixth term in the 11 April elections, with 79.32 percent of the vote.
Some experts have questioned whether his death was a coup d'état.
The African Union, which has a policy of suspending states when there is an unconstitutional change of power, issued a statement on Tuesday night in which it conveyed condolences.
"The late President Déby played a key role in promoting the African Union's agenda to promote peace and security and was an ardent supporter of the union's efforts to create an environment of security and stability, especially within the Sahel region," the statement read.
The AU "stands in solidarity with the government and people of the Republic of Chad during this difficult time and joins them in mourning the loss of a champion of Africa's endeavours to provide a peaceful, secure and stable continent".
AU Commission chairperson, Moussa Faki Mahamat, hails from Chad.