The U.S. Embassy in Qatar warned American citizens to "shelter in place until further notice." The embassy said in its June 23 alert that it was acting "out of an abundance of caution" but gave no other details. Qatar is home to the Al Udeid Air Base, the largest U.S. air base in the Middle East.

"Out of an abundance of caution we recommend American citizens shelter in place until further notice," the U.S. Embassy in Doha, Qatar, said Monday amid concerns about retaliation from Iran.