Sri Lanka's separatist Tamil Tigers have killed 31 troops in the latest fighting, a pro-rebel website said Saturday.  Tamil Tigers said they beat back an army advancement into rebel-held areas of Vavuniya and Mannar during several confrontations, killing 31 and wounding at least 52, government troops, the website report said.  "The Sri Lanka army pulled back after sustaining casualties," the rebels said, without giving their own casualty figures.