WASHINGTON, Aug 20 (Reuters) - U.S. President Joe Biden said on Friday a large evacuation of Americans and Afghan allies from Afghanistan is a risky operation and he cannot guarantee the final outcome. Biden, in remarks in the White House East Room, said the United States is in close contact with the Taliban to gain access to the Kabul airport for U.S.-aligned Afghan citizens. Reporting by Andrea Shalal and Steve Holland Editing by Chris Reese Our Standards: The Thomson Reuters Trust Principles.