South African says 13 of its soldiers were killed and 27 others wounded in fighting with rebels in the capital of the Central African Republic. President Jacob Zuma says that one soldier remains missing after the clashes that began Saturday in Bangui.
Seleka rebels took control of the city Sunday, forcing President Francois Bozize to flee.
Zuma says South Africa "rejects any attempt to seize power by force," and that the soldiers' deaths will not keep the country from working to prevent the overthrow of elected governments.
South Africa had sent 200 troops to help the Central African Republic's military.
United Nations Secretary-General Ban Ki-moon condemned what he called an "unconstitutional seizing of power" by the rebels, and urged a "swift restoration of constitutional order."
Ban says he is deeply concerned about reports of human rights abuses and looting in Bangui, "including of United Nations property."
He said an agreement between the government and the rebels signed in January remains the best way to ensure peace and security. That deal calls for Bozize to remain in power until his term ends in 2016, with an opposition member named prime minister.
The rebels, who began their offensive in December, accuse the president of breaking the agreement.
Bozize's location has not been made public. Some reports say he fled across the border into the Democratic Republic of Congo, but DRC officials say he is not there.
State Department spokeswoman Victoria Nuland says the United States is deeply concerned about the deteriorating situation in CAR. She urges the rebels to establish law and order and restore such basic services as water and electricity.
Bozize has led CAR since taking power in a 2003 coup. CAR has a history of coups and unrest since winning independence from France in 1960.
Seleka political spokesman Eric Massi told VOA French to Africa that President Bozize must leave CAR to bring peace.