Thirteen South African soldiers were killed in the Central African Republic as rebels seized the capital over the weekend, President Jacob Zuma has said.
Mr Zuma said the South Africans had died in a nine-hour "high-tempo battle" against the "bandits" in Bangui.
South Africa had about 200 troops stationed in the city to block Seleka rebels from seizing power.
Ousted CAR leader Francois Bozize has fled to neighbouring Cameroon, officials there have announced.
Following the rebel takeover, the African Union has suspended CAR and imposed sanctions on Seleka's leaders.
"The council asks all member states to take the measures necessary to completely isolate the authors of this anti-constitutional change of power," said AU peace and security council chief Ramtane Lamamra, adding that this includes travel restrictions and an asset freeze on Seleka's leaders.
Rebel leader Michel Djotodia said he was now in charge but would uphold a peace accord that promised elections in three years.
He also said the existing power-sharing government - headed by Prime Minister Nicolas Tiangaye, an opposition figure - would remain in place.
"We are not here to carry out a witch-hunt," Mr Djotodia told Radio France Internationale (RFI).
However, another rebel leader, Nelson N'Djadder, told Associated Press news agency that he did not recognise Mr Djotodia as the new leader of CAR.
"We had agreed that we would push to Bangui in order to arrest Bozize and that we would then announce an 18-month transition, a transition that would be as fast as possible and not one that would last three years," Mr N'Djadder told AP from Paris.
"I have enough soldiers loyal to me to attack Djotodia. I am planning to take the Wednesday flight to Bangui.''
Looters and armed gangs roamed the streets of Bangui after Mr Djotodia's forces captured the presidential palace on Sunday.
Tens of thousands of people have fled CAR into neighbouring Democratic Republic of Congo and Cameroon in recent days, the UN refugee agency (UNHCR) says.
Mr Zuma said just over 200 South African troops had battled more than 1,000 rebel fighters.
"They fought a high-tempo battle for nine hours defending the South African military base, until the bandits raised a white flag and asked for a ceasefire," he said.
"Our soldiers inflicted heavy casualties among the attacking bandit forces."
Thirteen South African troops were killed, 27 wounded and one was unaccounted for, Mr Zuma said.
It was the highest death toll South Africa's army had faced in a single battle since white minority rule ended in 1994, correspondents say.
The BBC's Milton Nkosi in Johannesburg says Mr Zuma's government must be worried that one of the most powerful armies in Africa had suffered heavy casualties at the hands of a rebel group.
It was bewildering that South Africa had sent troops to CAR as part of a bilateral agreement with Mr Bozize's government, rather than through the AU, he says.
The South African National Defence Force Union (Sandu), which represents soldiers, said Mr Zuma should recall troops immediately, "even if it entails launching a military offensive to relieve our troops in distress", its national secretary Pikkie Greeff said.
But Mr Zuma said South Africa planned to reinforce its troops in CAR, as there was "no reason" to leave.
South Africa sent 200 troops in January out of a planned deployment of 400, Mr Zuma said.
Last week, Seleka denounced the South African forces as "mercenaries" and demanded their withdrawal.
Mr Djotodia told RFI that free and fair elections would be held within three years, as outlined in the peace agreement signed with Mr Bozize in January.
The deal collapsed last week when Seleka withdrew its members from the government.
The group accused Mr Bozize of running a parallel administration and failing to honour key aspects of the deal, including the release of political prisoners.
UN Secretary-General Ban Ki-moon condemned the rebel seizure of power and called for "the swift restoration of constitutional order".
On Monday French troops guarding Bangui's international airport accidentally killed two Indian citizens and wounded a number of Cameroonians, the French defence ministry said.
The French soldiers shot at three vehicles approaching the airport after firing warning shots and themselves coming under fire, a statement said.
France doubled its military detachment at the airport to around 600 troops on Saturday in order to protect French citizens as rebels approached Bangui.
CAR, which has a population of about 4.5 million, has been hit by a series of rebellions since independence from France in 1960.