Chad's president has promised what he called an international inquiry into the disappearance of two opposition figures after this month's fighting.
Chad's government denies holding the missing men
Idriss Deby made the announcement after meeting his French counterpart, Nicolas Sarkozy, in the capital Ndjamena.
Mr Sarkozy said he would not give any ground in his call for a commission of inquiry to be set up.
Chad Foreign Minister Ahmad Allam-Mi said one of the men was not missing and would probably talk to the press later.
Human rights groups say the two, Ngarlejy Yorongar and Ibni Oumar Mohamat Saleh, were arrested in Ndjamena on 3 February as rebel fighters moved out of the city following several days of fighting with government troops.
Rights groups accuse the government of using the coup as a pretext to arrest the men, and have urged Paris to demand evidence that the men are alive.
Chadian authorities have admitted holding a third opposition figure.
Lol Mahamat Choua, who also went missing on 3 February, has been released from a military prison but is under house arrest, the government said on Tuesday.
The government accuses him of conspiring with the rebels who attempted the coup.
'France wants the truth'
Mr Sarkozy is stopping briefly in Chad as he travels to South Africa for an official visit.
After a meeting with Mr Deby, he said Chad could not act with impunity.
"France wants the truth and and I won't give ground on this point," he said.
"Just because the government of Chad is legitimate it cannot allow itself to do whatever it likes. I have demanded a commission of inquiry."
On Tuesday, Mr Allam-Mi was in New York, meeting UN Secretary General Ban Ki-moon to discuss issues including restarting a process of political dialogue with rebels not involved in the coup attempt, and the disappearance of the opposition figures.
He said he had received information from Mr Yorongar's sister and chauffeur that he had gone into hiding of his own accord.
But AFP news agency quoted the chauffeur, Saleh Dkekotar, as saying: "We've had no news, he's not hiding. If he were alive he would come directly to the house."
In a detailed report, Human Rights Watch quotes witnesses as saying Mr Yorongar and Mr Saleh were taken by government soldiers on 3 February.
A spokesman for the campaign group, Reed Brody, told news agency Reuters that Mr Sarkozy should not visit Chad without first receiving proofs that the two men were alive.
The coup attempt left hundreds dead
"Given there is little doubt they were taken by the authorities, the authorities should produce them," Mr Brody said.
France - Chad's former colonial ruler - maintains a force of some 1,500 troops in the country.
It provided Chad's army with logistical support in fighting off the rebels during the two-day coup attempt, which left the Chadian capital severely damaged, with bodies littering the streets.
Amnesty International says Paris learned that the three opposition figures had been detained eight days after the coup attempt, but did not disclose the information.
The rights group has urged France to reveal all it knows, and not to "cover up the excesses of the Chadian government".