The State Department cited the potential for terrorist attacks in the Middle East, North Africa, and the Arabian Peninsula.
"Current information suggests that al-Qaida 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 department said in a statement.
The department said that the alert does not expire until Aug. 31.
On Thursday, the State Department said it would close at least 18 diplomatic posts in the Middle East and surrounding areas because of the threats. On Friday, it confirmed it would close 21 posts in the region.
The full release from the State Department is below:
The Department of State alerts U.S. citizens to the continued potential for terrorist attacks, particularly in the Middle East and North Africa, and possibly occurring in or emanating from the Arabian Peninsula. Current information suggests that al-Qa'ida 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. This Travel Alert expires on August 31, 2013.
Terrorists may elect to use a variety of means and weapons and target both official and private interests. 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. U.S. citizens should take every precaution to be aware of their surroundings and to adopt appropriate safety measures to protect themselves when traveling.
We continue to work closely with other nations on the threat from international terrorism, including from al-Qa'ida. Information is routinely shared between the U.S. and our key partners in order to disrupt terrorist plotting, identify and take action against potential operatives, and strengthen our defenses against potential threats.
We recommend U.S. citizens register their travel plans with the Consular Section of the U.S. Embassy through the State Department's travel registration website. We strongly recommend that U.S. citizens Traveling abroad enroll in the Department of State's Smart Traveler Enrollment Program (STEP). STEP enrollment gives you the latest security updates, and makes it easier for the U.S. embassy or nearest U.S. consulate to contact you in an emergency. If you don't have Internet access, enroll directly with the nearest U.S. embassy or consulate.
For the latest security information, U.S. citizens traveling abroad should regularly monitor the Department of State's Internet website at travel.state.gov where the Worldwide Caution, Country Specific Information, Travel Warnings, and Travel Alerts can be found. Follow us on Twitter and the Bureau of Consular Affairs page on Facebook as well. Download our free Smart Traveler app, available through iTunes or Google Play, to have travel information at your fingertips.
In addition to information on the internet, travelers may obtain up-to-date information on security conditions by calling 1-888-407-4747 toll-free in the United States and Canada or, from other countries, on a regular toll line at 1-202-501-4444. These numbers are available from 8:00 am to 8:00 pm Monday through Friday, Eastern Time (except U.S. federal holidays).