WASHINGTON—Al Qaeda re-emerged as a top global security threat after suspected plots by an affiliate of the terror group led the State Department to issue a world-wide travel alert for the entire month of August.

Senior U.S. officials said they were particularly focused on Yemen-based al Qaeda in the Arabian Peninsula, or AQAP, saying the affiliate was plotting attacks that Washington feared could be executed in the Middle East, Africa or beyond.

These officials cited increased communications, or "chatter," between terrorist operatives in the field as the primary reason behind the State Department's alert. The Obama administration said on Thursday that it would close most of its embassies in the Middle East on Sunday because of the threat.

"Current information suggests that al Qaeda and affiliated organizations continue to plan terrorist attacks both in the region and beyond, and that they may focus efforts to conduct attacks in the period between now and the end of August," the State Department said in a statement on Friday.

The warning didn't tell travelers to abandon their journeys, advising caution and recommending that U.S. citizens register their travel plans on the department's website.