TRIPOLI (Reuters) - The United States evacuated its embassy in Libya on Saturday, driving its staff under heavy military guard across the border to Tunisia after escalating clashes broke out between rival militias in Tripoli. Security in the Libyan capital has deteriorated following two weeks of fighting between brigades of former rebel fighters who have exchanged rocket, can**n and artillery fire in southern Tripoli near the embassy compound. The violence is the worst seen in Tripoli since the 2011 fall of Muammar Gaddafi.