SANAA, Yemen — Fighters for Yemen's largest tribe sealed off key government buildings and barricaded streets in the heart of the capital Tuesday as the revolt against President Ali Abdullah Saleh sharply escalated after militiamen turned their guns against government forces.The tribal fighters appeared to consolidate their control of a key district in Sanaa – which includes ministries and the ruling party headquarters – after the fiercest clashes in the three-month uprising against Saleh's authoritarian rule.