The State Department says the potential for terrorism is particularly strong in the Middle East and North Africa. It says an attack could occur or come from the Arabian Peninsula.
Spokeswoman Marie Harf cited information indicating a threat to U.S. facilities overseas and said some diplomatic facilities may stay closed for more than a day.
Other U.S. officials said the threat was in the Muslim world, where Sunday is a workday. American diplomatic missions in Europe, Latin America and many other places are closed on Sunday.