French President Nicolas Sarkozy has condemned French charity workers for trying to take more than 100 children out of the Chad-Darfur border region.
Chadian President Idriss Deby visited the children on Friday
Mr Sarkozy branded their actions as illegal and unacceptable.
Chad is holding nine French nationals and the Spanish crew of a chartered plane in the eastern town of Abeche on suspicion of kidnapping.
The charity has said the children are Darfur orphans, but UN workers believe many are from villages in Chad.
More than 230,000 refugees from Sudan's war-torn Darfur region live in Chad.
Mr Sarkozy called his Chadian counterpart Idriss Deby to discuss the actions of the French charity, Zoe's Ark.
Mr Deby has promised "severe punishment" for what he has described as a "kidnapping" or "child-trafficking" operation.
"Their aim is to kidnap the children from their parents, to steal the children from their parents and sell them to paedophile organisations in Europe, and even perhaps to kill them and sell their organs," Mr Deby said.
The French Secretary of State for Foreign Affairs and Human Rights, Rama Yade, said: "I can understand the families, the French families who wanted to save children. But I don't understand why an association decided, alone, to bring them to Paris. That's why we completely disapprove of this initiative."
Denying it planned to sell the children for adoption, Zoe's Ark says it was given statements from tribal leaders that all the children were Darfur orphans with no known relatives.
The charity insists it was trying in good faith to take endangered children abroad for medical treatment.
However, a BBC reporter says the children appear to be in good health.
On the runway
Staff from the UN children's agency Unicef say many of the children, now being kept in an orphanage in Abeche, cry at night for their parents and say they are from villages in Chad.
The workers say they were assured that the children were orphans
The BBC's Stephanie Hancock was among a group of reporters taken by Chadian authorities to the airport at Abeche, a town close to the Sudanese border, and shown the private charter plane still sitting on the runway where it was abandoned three days ago.
The reporters were later taken to the local police headquarters to see the 16 detained Westerners - six French charity-workers, three French journalists and a seven-strong Spanish crew.
They are not being held in prison cells but in a large room and are showing no signs of mistreatment, our correspondent says.
The seven Spaniards among the group are the plane's two pilots and five air stewards.
Spanish media have reported they are employees of the Barcelona-based charter company, Girjet. The company said it had provided transport for the charity but was not otherwise involved in the plan, media reported.
It is still unclear whether charges will be brought against the Europeans.
The reporters were also taken to the orphanage where the 103 children are being cared for by aid workers and UN staff.
Aid workers confirmed they were not treating any of the children for any serious illnesses or injuries.
The vast majority of the children are believed to be between three and five years old, with the oldest about eight or nine, and several babies no more than one and a half, our correspondent says.
The president of the French national committee for Unicef said 48 of the children questioned so far appeared to be Chadian, not Sudanese.
"Our impression is that the majority aren't orphans, but at this stage it's just an impression," Jacques Hintzy told Radio Television Luxembourg.
A Paris court began investigating the charity last Tuesday after receiving a report about the unauthorised action.