Chad's government and four Sudan-based Chadian rebel groups signed a "definitive peace accord" in Libya on Thursday that included an immediate ceasefire, a Chadian presidency official said. The deal, which aimed to end more than two years of sporadic fighting in eastern Chad, was signed in the Libyan city of Sirte in the presence of Chadian President Idriss Deby, Sudanese President Omar Hassan al-Bashir and Libyan leader Muammar Gaddafi, the official, who asked not to named, told Reuters.