The order impacts Americans in Bahrain, Egypt, Iran, Iraq, Israel and Gaza, Jordan, Kuwait, Lebanon, Oman, Qatar, Saudi Arabia, Syria, United Arab Emirates and Yemen.
The U.K. has authorized American use of its bases for Iran missile operations, while the Royal Air Force intercepts Iranian drones threatening Qatar and Cyprus.
The drone is thought to be the first taken down by a British fighter jet since the US and Israel began striking Iran on Saturday, prompting furious retaliation from Tehran.
The UK has entered the Iran conflict, with an RAF Typhoon jet shooting down an Iranian drone near Qatar, marking a significant shift in Britain's military stance.