Four Chadian rebel groups initialled a peace agreement with the government on Wednesday at talks in Libya, a Chadian official said, but the leader of the main faction said there were many points left to resolve. "The contents are secret. An agreement should be officially signed very soon in a ceremony that will bring together heads of state in Tripoli," a senior Chadian government official, who asked not to be named, told Reuters in Chad's capital N'Djamena.