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.

The U.S. embassy in Qatar emailed a message to U.S. citizens in the Gulf country on Monday recommending that they shelter in place until further notice.