The daughter of detained former Ivory Coast leader Laurent Gbagbo has written to French President Nicolas Sarkozy voicing concern over his fate, one of her lawyers said on Thursday.

"My father, mother and brother have been arrested, not to say kidnapped as war booty, without any legal mandate," wrote lawyer Marie-Antoinette Singleton, who lives in the United States, according to her counsel Gilbert Collard.

Gbagbo was captured by forces loyal to new President Alassane Ouattara on April 11, after nearly five months of a bitter election feud sparked by his refusal to relinquish power despite being deemed to have lost to Ouattara.

Gbagbo, his wife Simone and dozens of others were arrested during a military operation in the main city Abidjan. The former president and his wife are under house arrest in separate towns in the former rebel north of the country.

"They have been imprisoned and held in a secret location, without knowing what charges have been made against them," Singleton wrote.

"Showing contempt for the most basic humanity, the family has no news of them," Singleton wrote, adding that she had had no response to a request she made to Ouattara to be allowed to visit her father.

Ouattara's government has begun steering the crisis-hit west African country, the world's top cocoa producer, back to normality but still faces resistance from pro-Gbagbo fighters who have refused to disarm.

Ouattara's government has launched an invesigation into the alleged crimes of Gbagbo's regime.