The State Department issued a worldwide travel alert Friday, warning of potential terrorist attacks by al-Qaida and its affiliates in the Middle East and North Africa that could target tourists on trains, flights or other forms of public transportation.
The alert follows the decision to close 21 U.S. embassies across the Muslim world on Sunday in response to the same security threat, according to State Department officials.
"Current information suggests that al-Qaida 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 State Department said in a statement Friday. It said the potential for attacks was particularly high in the Middle East and North Africa and that it could come from or occur on the Arabian Peninsula.
U.S. citizens traveling abroad were urged to take precautions. The alert warned that "terrorists may elect to use a variety of means and weapons and target both official and private interests," notably public transportation systems including "subway and rail systems, as well as aviation and maritime services."
Speaking at a news briefing Thursday, State Department spokeswoman Marie Harf said a number of embassies were instructed to close on Sunday because of "security considerations." Sunday is a normal working day in most Muslim countries, and embassies there would typically be open for business.
Harf stressed that officials were acting "out of an abundance of caution and care for our employees and others who may be visiting our installations." She did not provide details of the security threat but said that embassies could remain closed into next week.
The last time the department issued a similar warning was on last year's anniversary of the Sept. 11, 2001, attacks. The U.S. ambassador to Libya and three other Americans were killed Sept. 11 and 12, 2012, when militants assaulted two U.S. compounds in Benghazi.
Rep. Edward Royce, R-Calif., chairman of the House Foreign Affairs Committee, told CNN's "New Day" program: "It's my understanding that it is al-Qaida-linked, all right."
Among the affected embassies are those in Afghanistan, Iraq, Kuwait, Saudi Arabia and Libya. Three consulates will also close - two in Saudi Arabia and one in the United Arab Emirates.