France expressed disappointment Tuesday at Mexico's refusal to let a Frenchwoman serve out a 60-year sentence for kidnapping in her native country, while her lawyer said she would keep appealing until the case reaches an international court.

Mexican President Felipe Calderon announced Monday that his government decided against returning Florence Cassez because France would not guarantee she would serve the full sentence.

French Foreign Minister Bernard Kouchner said he was "very upset" and still hoped for a solution that would allow Cassez to serve out her sentence in France.

"We had a working group, of jurists, both French and Mexican who were meant to find a solution because the two judicial systems are not the same," he said. "I hope that with the appeal, Florence will be able to finish her term in France," he said.

Cassez has acknowledged she lived at a ranch near Mexico City where three kidnap victims were held, including an 8-year-old girl. But she said she was simply dating a Mexican arrested in the case and did not know the people at the ranch had been kidnapped.

One of the victims, however, identified Cassez as one of her captors, and another suspect in the case said that the Frenchwoman not only participated in abductions, but helped lead the gang that carried them out.

With Mexico refusing to send her back, Cassez will prepare the last appeal she has left in Mexican courts, said her defense attorney, Agustin Acosta.

He said he did have much hope that the appeal would be successful, and Cassez will likely take her case to the Inter-American Commission of Human Rights, a branch of the Organization of American States.

In a process that could take years, the commission would then decide whether the case should be heard by the Costa Rica-based Inter-American Court of Human Rights.

Cassez's imprisonment became a hotly debated issue in France after Mexican police acknowledged they re-staged her capture for news media. French President Nicolas Sarkozy visited Mexico in March and persuaded Mexican officials to study the possibility of returning Cassez to France.

France's request stirred controversy in Mexico, where anti-crime activists and politicians have been demanding tougher sentences to stamp out a wave of kidnappings.