The US state department has said it is issuing a global travel alert because of an unspecified al-Qaeda threat.
In a statement, the department said the potential for an attack is particularly strong in the Middle East and North Africa.
The move comes shortly after the US announced the closure of embassies in the region on Sunday.
The alert expires on 31 August 2013, the department said. It recommended US citizens travelling abroad be vigilant.
"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," the statement said.
An unnamed senior US official told NBC the threat may be related to the Islamic holy month of Ramadan, which ends next week.
Among the US embassies affected by the closures on Sunday are Israel, Egypt, Turkey, Algeria, Yemen, Saudi Arabia and Bangladesh.
"It is possible we may have additional days of closing as well," a senior state department official told the BBC earlier.
The official said the state department had "been apprised of information" leading it to take these "precautionary steps... out of an abundance of caution".
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How can traveler's be vigilant? If they're going to attack, you're not going to be able to stop them.