The U.S. State Department has issued a global travel alert because of an unspecified al Qaeda threat.

According to BBC News, in a statement, the department said the potential for an attack was particularly strong in the Middle East and North Africa.

It comes shortly after the U.S. announced nearly two dozen embassies and consulates would be shut on Sunday, Aug 4.

The alert expires on August 31, 2013, the Department said. It recommended U.S. 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.

The alert warned of "the potential for terrorists to attack public transportation systems and other tourist infrastructure".

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.