The ousted president of the Central African Republic (CAR) has accused Chad of helping the rebels that toppled him.
Francois Bozize told the BBC it was "Chadian special forces" who led the final operation of the rebellion on the weekend of 23-24 March.
Chad, which is hosting an emergency regional summit on CAR, has not commented on the allegations.
Mr Bozize, who has fled to Cameroon, said he requested but was refused a seat at the meeting.
But South Africa's President Jacob Zuma is one of those attending the Economic Community of West African States (Ecowas) summit.
Thirteen South African soldiers were killed in the clashes in the capital, Bangui, when the Seleka rebel group seized power after a peace deal negotiated with Mr Bozize collapsed.
The incident has caused controversy in South Africa. The governing ANC party has rejected accusations that the troops were there because of mining interests.
South Africa's government says they were training government forces and providing security.
"On Saturday March 23 we had neutralised Seleka forces but overnight into Sunday 24, we knew that there had been support from an African country, which I inevitably believe was Chad," Mr Bozize told the BBC French Service.
"We can confirm it was Chadian special forces that led the operation on the Sunday morning and attacked the barracks of the South Africans," he said.
"To avoid a bloodshed I decided to resign after a phone call I made to the French ambassador."
Mr Bozize, who seized power in a coup in 2003 and won two subsequent elections, said he was surprised by Chad's move.
"We had a solid and friendship relation with the Chadian authorities. Chad alone can give an explanation," he said.
Louisa Lombard, a fellow at the University of California in Berkeley who specialises in the CAR, said relations between the neighbours had soured in recent month.
Up until last year, Chad's President Idriss Deby provided Mr Bozize with his personal guards, she said.
"One of the things that really frustrated President Deby was the presence of a Chadian rebel group on Central African territory and Deby's sense that Bozize didn't do enough to deal with this group," Ms Lombard told the BBC's Newsday programme.
"Another thing was probably that President Bozize was starting to pursue alliances with other countries, notably South Africa - and that this was seen a compromising somewhat his relationship with Deby," she said.
Michel Djotodia, the rebel leader who has declared himself president, is sending his Prime Minister Nicolas Tiangaye to the Ecowas summit in Chad's capital, N'Djamena.
Mr Djotodia announced a caretaker government on Sunday and says he will run the country until elections in 2016.