KABUL, Aug 18 (Reuters) - Seventeen people were injured on Wednesday in a stampede at a gate to the airport in the Afghan capital, Kabul, a NATO security official said, as Western countries stepped up the evacuation of their diplomats and others. Afghan civilians seeking to leave after the Taliban seized the capital on Sunday had been told not to gather around the airport unless they had a passport and visa to travel, said the official, who was working at the airport. The official, who declined to be identified, said he had not heard any reports of violence by Taliban fighters outside the airport. Reporting by Kabul bureau; Editng by Robert Birsel and Alex Richardson Our Standards: The Thomson Reuters Trust Principles.