More than 20 U.S. diplomatic missions across the Islamic world have closed for the day in response to fears of unspecified attacks by militants.
U.S. embassies and consulates across the Middle East, North Africa, and parts of Asia were closed on August 4, a working day in many Muslim countries.
Embassies affected included those in Iraq, Egypt, Afghanistan, and Yemen.
Reports said measures were particularly strict in the Yemeni capital, Sanaa, where Britain, France, and Germany also closed their embassies.
The U.S. State Department has said embassies could be closed on some other days as well.
On August 3, the White House held a high-level meeting on terrorism concerns that prompted Washington to issue a global travel warning in August, citing unspecified plans by Al-Qaeda to strike U.S. interests in the Middle East or North Africa.
The August 3 meeting was held as Interpol issued a global security alert over the escape of hundreds of Al-Qaeda militants in jailbreaks during the past month.
The jailbreaks were in nine countries, including Iraq, Libya, and Pakistan.
U.S. President Barack Obama did not attend the White House meeting, but was briefed afterward.
Those attending included national security adviser Susan Rice, U.S. Secretary of State John Kerry, Secretary of Defense Chuck Hagel, and Homeland Security Secretary Janet Napolitano.
Based on reporting by AFP, Reuters, AP, and BBC