KATMANDU, Nepal — An avalanche swept down a climbing route on Mount Everest early Friday, killing at least 12 Nepalese guides and leaving four missing in the deadliest disaster on the world's highest peak. Several more were injured. The Sherpa guides had gone early in the morning to fix ropes for other climbers when the avalanche hit them at about 6:30 a.m., Nepal Tourism Ministry official Krishna Lamsal said from the base camp where he is monitoring rescue efforts.