WASHINGTON, Aug 16 (Reuters) - U.S. forces protecting the airport in Kabul have killed two gunmen in separate incidents, a U.S. official told Reuters. The official, speaking on condition of anonymity, said that the incidents had occurred over the past 24 hours. "Initial reports indicate armed assailants fired into the crowd," the official said. Reporting by Idrees Ali Editing by Chizu Nomiyama and Franklin Paul Our Standards: The Thomson Reuters Trust Principles.