WASHINGTON, Aug 18 (Reuters) - The number of U.S. troops at Kabul"s international airport reached about 4,500 on Wednesday, a U.S. official said. The official, speaking on condition of anonymity, said that the number is set to hit 6,000 in the coming days. Reporting by Idrees Ali Editing by Chizu Nomiyama Our Standards: The Thomson Reuters Trust Principles.