KABUL, Aug 20 (Reuters) - More than 18,000 people have been evacuated from Kabul airport since the Taliban took over the Afghanistan capital, a NATO official told Reuters on Friday. However, crowds continued to throng outside the airport, desperate to flee, said the official, who declined to be identified. The Taliban took over Kabul on Sunday. Reporting by Rupam Jain, Writing by Raju Gopalakrishnan; Editing by Himani Sarkar Our Standards: The Thomson Reuters Trust Principles.