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”.