CAIRO, April 2 (Reuters) - Unidentified gunmen killed five African Union peacekeepers in the Darfur region of western Sudan, the heaviest death toll suffered by the African force in one day since late 2004, a spokesman said on Monday. The five were guarding a water point near the Sudanese border with Chad when they came under fire on Sunday, Noureddine Mezni told Reuters. Four soldiers were killed in the shooting and the fifth died of his wounds on Monday morning. Three gunmen were also killed, he said.