President Nicolas Sarkozy travels to Chad on Wednesday as rights groups urged France not to "cover up" for President Idriss Deby Itno, accused of having a hand in the disappearance of opposition members.
The president will make a brief stopover in Ndjamena en route to South Africa, some three weeks after France helped Chadian forces repel an attack from rebels backed by Sudan.
With his approval rating at an all-time low at home, Sarkozy embarks on the Africa trip with his wife Carla Bruni who will be making her first trip abroad as France's new first lady.
Sarkozy will press Deby to set up a commission of inquiry into the disappearance of opposition members Ngarlejy Yorongar and Ibni Oumar Mahamat Saleh, his spokesman David Martinon said.
He will sit down for dinner with French troops who earlier this month provided fuel, food, aerial intelligence and Libyan ammunition to the Chadian army to help them push back the rebel offensive.
The visit comes as troops from a French-led EUFOR mission to protect refugees from Sudan's troubled Darfur region are arriving in eastern Chad and the Central African Republic.
Preserving stability in Chad is vital to the success of the EU force after its deployment was briefly suspended during fighting this month.
"We are going there because the deployment of the EUFOR forces is at sake," said the French minister for cooperation, Jean-Marie Bockel.
But he added that Sarkozy will also exert "extremely strong pressure" on Deby to ensure the rebel assault does not provide the leader with an excuse to clamp down on dissent and step off the path toward democracy.
US-based Human Rights Watch and Amnesty International urged France this week to press Deby to shed full light on the fate of the two opposition deputies who went missing on February 3.
"Our inquiry leaves little doubt that it was the government which took them and we hold the government fully responsible for their well-being and safe return," said Georgette Gagnon, Africa director at Human Rights Watch.
"These men are at grave risk of being tortured. The French government should not cover up the excesses of the Chadian government -- whatever bilateral agreements they may have," said Tawanda Hondora from Amnesty International.
Chadian Foreign Minister Ahmad Allam-Mi said in New York on Tuesday that Yorongar was in Ndjamena and would probably "explain the circumstances of his arrest" on Wednesday but there was no word on Ibni, the spokesman of Chad's main opposition coalition.
Sarkozy's visit to South Africa on Thursday and Friday is expected to focus on energy cooperation, with France's nuclear giant Areva bidding against US-based Westinghouse Electric for the construction of nuclear reactors.
He is to deliver an address to parliament in Cape Town that will be closely followed after a speech he gave in Dakar in July was condemned as racist.
In the Dakar speech, Sarkozy said that Africans had turned their backs on progress and that colonialism should not be blamed for all of Africa's ills.
In Johannesburg on Friday, Sarkozy and Bruni are to hold a private meeting with Nelson Mandela, South Africa's 89-year-old anti-apartheid icon.
The visit to South Africa will be Sarkozy's first to an English-speaking African country and underscores his administration's desire to establish strong ties with the continent's economic powerhouse.
President Thabo Mbeki had a frosty relationship with Sarkozy's predecessor Jacques Chirac who had criticised the South Africa's peace initiatives in Ivory Coast.