Libya says it is building its case against Muammar Qaddafi's detained son, gathering witnesses and documents, according to the Hague-based war crimes prosecutor, as it seeks to persuade the International Criminal Court to allow for a local trial.

The ICC issued an arrest warrant for Saif al-Islam last year after prosecutors accused him and others of involvement in the killing of protesters during the revolt that eventually toppled his father, who ruled with an iron fist for 42 years.