The US state department has issued a global travel alert because of fears of an unspecified al-Qaeda attack.
The department said the potential for an attack was particularly strong in the Middle East and North Africa.
The US intercepted electronic communications between senior al-Qaeda figures, according to officials quoted by the New York Times.
The alert comes shortly after the US announced nearly two dozen embassies and consulates would be shut on Sunday.
The US state department said the alert expires on 31 August 2013 and it recommended US citizens travelling abroad be vigilant.
"Current information suggests that al-Qaeda and affiliated organisations 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.
The alert warned of "the potential for terrorists to attack public transportation systems and other tourist infrastructure".
In its report, the New York Times says high-level intercepts were collected and analysed this week and that the CIA, state department and White House immediately recognised their significance.
Meanwhile, the UK Foreign Office has confirmed that the British embassy in Yemen is to be closed on Sunday and Monday as "a precautionary measure".
'Not chit chat'
US President Barack Obama has ordered that "all appropriate steps" be taken to protect Americans in response to a threat of an al-Qaeda attack, AFP news agency quoted a White House official as saying.
"The president is being updated on a potential threat occurring in or emanating from the Arabian Peninsula," the official added.
Nancy Pelosi, the Democratic leader in the House of Representatives, said that congressional leaders had been briefed about the alert.
"There is some understanding of the seriousness of the threat," she told reporters.
Republican lawmaker Jason Chaffetz said he understood there was "a very real worldwide threat".
Dutch Ruppersberger, the top Democrat on the House intelligence committee, said the threat was not prompted by "the regular chit chat" gleaned from would-be militants online or elsewhere.
"The most important thing we have to do is protect American lives," he told the Associated Press news agency.
'Abundance of caution'
An unnamed senior US official told NBC the threat may be related to the Islamic holy month of Ramadan, which ends next week.
In the Muslim world, Sunday is a work day. In other parts of the world US diplomatic offices are shut on Sunday.
"It is possible we may have additional days of closing as well," a senior state department official told the BBC on Thursday.
The official said the state department had "been apprised of information" leading it to take these "precautionary steps... out of an abundance of caution".
Last year on 11 September, the US consulate in Benghazi, Libya, was attacked, leaving four Americans dead.
Other embassies are routinely targets of protesters.
The US diplomatic missions to be closed on Sunday are in Abu Dhabi, United Arab Emirates; Algiers, Algeria; Amman, Jordan; Baghdad, Iraq; Cairo, Egypt; Dhahran, Saudi Arabia; Djibouti, Djibouti; Dhaka, Bangladesh; Doha, Qatar; Dubai, United Arab Emirates; Irbil, Iraq; Jeddah, Saudi Arabia; Kabul, Afghanistan; Khartoum, Sudan; Kuwait City, Kuwait; Manama, Bahrain; Muscat, Oman; Nouakchott, Mauritania; Riyadh, Saudi Arabia; Sanaa, Yemen and Tripoli, Libya.