The workers say they were assured that the children were orphans

Ahmat Mahamat Bachir told the BBC that a judge was expected to lay charges of child abduction against the workers.

Ten other people have been detained, including seven Spanish crew of the plane that was to be used by the charity, known as Zoe's Ark.

The charity has denied it planned to sell the children for adoption.

It has said the 103 children are orphans from Sudan's war-torn Darfur region.

France is a good mother, we will be with these French nationals to protect them as far as we can

Rama Yade, French Secretary of State for Foreign Affairs and Human Rights

Children's trauma Profile: Zoe's Ark

However 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.

Mr Bachir said the case would go before a judge on Monday.

"They made fake visas, which means they forged the documents. For us, abduction is more than a crime. They could be put in jail for several years.

"They committed the offence in Chad, so they would be imprisoned in Chad of course, it's very probable," he said.

The French Secretary of State for Foreign Affairs and Human Rights, Rama Yade, said the planned operation by the charity was "irresponsible" but that France would offer its citizens "maximum consular assistance".

The children are not being treated for any serious illnesses or injuries

"France is a good mother, we will be with these French nationals to protect them as far as we can, to guarantee their rights and we will never leave them," she told Europe 1 radio.

French President Nicolas Sarkozy has said the charity workers' actions were illegal and unacceptable.

Chad's President Idriss Deby has promised "severe punishment" for what he has described as a "kidnapping" or "child-trafficking" operation.

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

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, reports said.

'Chadian children'

The reporters were also taken to the orphanage where the 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.