Washington (CNN) -- Air travelers aren't likely to notice visible security changes as a result of a terrorism threat alert issued Friday by the State Department.
The extraordinary global travel alert advised that al Qaeda may launch attacks in the Middle East, North Africa and beyond in coming weeks, and the U.S. government prepared to close 21 embassies and consulates from Algeria to Bangladesh on Sunday as a precaution.
Three sources told CNN that the United States has information that Yemen-based al Qaeda in the Arabian Peninsula was in the final stages of planning for an unspecified attack.
"U.S. citizens are reminded of the potential for terrorists to attack public transportation systems and other tourist infrastructure. Terrorists have targeted and attacked subway and rail systems, as well as aviation and maritime services," the State Department's alert said.
Airlines said they haven't changed operations as a result of the alert.
The alert is focused on U.S. citizens traveling abroad, and the State Department suggested that travelers register with its Smart Traveler Enrollment Program, also known as STEP, which allows them to receive updated security information.
Airlines for America (A4A), the airline industry trade group, said is monitoring the situation and suggested that passengers could also sign up for travel notifications from their airlines
Delta Air Lines, which has one daily roundtrip flight between JFK Airport and Tel Aviv and one between Atlanta and Dubai, said it is operated with "Business as usual, for now."
United Airlines, which flies to North Africa and the Middle East, says there have been no flight schedule changes as the result of the warning. It says it is monitoring the situation and keeping in regular contact with the Transportation Security Administration and State Department. It isn't offering waivers for travelers wanting to change their flight, but that policy could change as the airline receives additional information.
American Airlines said it has no flights to Africa or the Middle East, "therefore we have nothing to waive."