Six French aid workers detained in Chad on suspicion of trying to illegally fly 103 children to Europe have gone on a hunger strike to protest what they called a "biased" probe into the incident, a judicial source has said.

"They have begun their hunger strike, but are continuing to drink water and smoke," the source told the AFP news agency.

"They are saying they are determined to continue their strike until they are freed because they say they have committed no crime."

The members of Zoe's Ark, a French charity, were arrested in Chad on October 25 as they tried to fly the children, aged 1 to 10, out of the central African country.

The source also said the group had begun the protest because they felt that no one was listening to their case, that they had been abandoned by the French government and that a Chadian official involved in the case had not been arrested.

Chadian trial

He also said it had been decided that the trial of the six, who face charges of fraud and abduction and could be sentenced to forced labour terms if convicted, would be held in Chad and that it would start in the coming weeks.

Neither lawyers for the French nationals nor Chadian court officials were immediately available to comment.

Zoe's Ark had said it wanted to fly orphans from Sudan's Darfur region to Europe for fostering by families but UN officials who questioned the children said most were not orphans and came from villages on the Chad-Sudan border.

The seven-member Spanish crew of a chartered plane, three French journalists and a Belgian pilot arrested with the group were later released after Nicolas Sarkozy, the French president, flew to Chad to discuss the case with Idriss Deby, his Chadian counterpart. 

France has strongly condemned the Zoe's Ark operation but the case has strained relations with its former colony ahead of the planned deployment of an EU peacekeeping force in Chad's restive east.