KABUL, Aug 17 (Reuters) - Military flights evacuating diplomats and civilians from Afghanistan started taking off on Tuesday morning, a Western security official at Kabul airport told Reuters. The airport runway and tarmac, overrun on Monday by thousands of people desperate to flee from the Afghanistan capital, are now clear of crowds, the official said. U.S. forces, which are in charge at the airport, had halted the evacuation flights because of the chaos. Reporting by Rupam Jain, Writing by Raju Gopalakrishnan; Editing by Christian Schmollinger Our Standards: The Thomson Reuters Trust Principles.