(CNN) -- Six French charity workers sentenced in Chad to eight years of hard labor for trying to kidnap children were on their way back to France on Friday, the French president's office said.
The six left the Chadian capital of N'Djamena Friday afternoon and were being flown back to Paris, the office said.
The transfer followed a letter from the French government and a phone call from French President Nicolas Sarkozy on Thursday requesting that the Chadian government allow the six to serve their sentences in France, the office said.
Such transfers are permitted under a 1976 judicial agreement between the two countries, a spokesman for the French Foreign Ministry said.
The six members of the France-based Zoe's Ark aid group were sentenced Wednesday after being found guilty of trying to kidnap 103 children from Chad in October. They were arrested as they tried to leave the country with the children.
The group claims it was on a mission to rescue orphans from Sudan's war-torn Darfur region, but other charities then investigated and determined most of the children were from Chad, not Sudan, and had at least one living parent. 
Watch a report on how the events unfolded »
Bilateral legal agreements require the aid workers to serve the length of their sentences, eight years, in a France prison, said Amandine Martin, a spokeswoman for the French Justice Ministry. Hard labor sentences don't exist in France.
Martin said that on arrival in France, the aid workers would be taken immediately to a public prosecutor and then to a criminal court, which would decide how the prisoners would serve their time.
The incident has been an embarrassment for France and strained its relationship with its former colony. French President Nicolas Sarkozy has personally intervened, and tried unsuccessfully to have the six workers put on trial in France. E-mail to a friend 
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