WASHINGTON, Aug 25 (Reuters) - The U.S. embassy in Kabul advised U.S. citizens not to travel to the Hamid Karzai International Airport at this time, according to an embassy security alert issued on Wednesday. Citizens already at the airport"s Abbey Gate, East Gate, and North Gate were advised to leave immediately, said the security alert. The alert, posted on the embassy website, gave no reason for why it was issued. It followed warnings by U.S. President Joe Biden and other administration officials of a threat by Islamic State to evacuation operations as crowds thronged airport gates, desperate to leave following the Taliban takeover of Afghanistan. The alert advised Americans to "be aware of your surroundings at all times, especially in large crowds."