The State Department warning urged American travelers to take extra precautions overseas, citing potential dangers involved with public transportation systems and other prime sites for tourists and noting that previous terrorist attacks have centered on subway and rail networks as well as airplanes and boats. It suggested travelers sign up for State Department alerts and register with U.S. consulates in the countries they visit.
The statement said that al-Qaida or its allies might target either U.S. government or private American interests. The alert expires on Aug. 31.
The State Department said the potential for terrorism was particularly acute in the Middle East and North Africa, with a possible attack occurring on or coming from the Arabian Peninsula.
U.S. officials pointed specifically to Yemen, the home of al-Qaida's most dangerous offshoot and the network blamed for several notable terrorist plots on the United States, from the foiled Christmas Day 2009 effort to bomb an airliner over Detroit to the explosives-laden parcels intercepted the following year aboard cargo flights.
"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," a department statement said.
The alert was posted a day after the U.S. announced it would shut many diplomatic facilities Sunday. Spokeswoman Marie Harf said the department acted out of an "abundance of caution" and that some missions may stay closed for longer than a day. Sunday is a business day in Muslim countries, and the diplomatic offices affected stretch from Mauritania in northwest Africa to Afghanistan.
"I don't know if I can say there was a specific threat," said Rep. Eliot Engel of New York, the House Foreign Affairs Committee's top Democrat, who was briefed on the State Department's decision. "There is concern over the potentiality of violence."
Although the warning coincided with "Al-Quds Day," the last Friday of the Islamic month of Ramadan when people in Iran and some Arab countries express their solidarity with the Palestinians and their opposition to Israel, U.S. officials played down any connection. They said the threat wasn't directed toward a specific American diplomatic facility.
The concern by American officials over the Yemen-based al-Qaida in the Arabian Peninsula is not new, given the terror branch's gains in territory and reach during Yemen's prolonged Arab Spring-related instability.
The group made significant territorial gains last year, capturing towns and cities in the south amid a power struggle in the capital that ended with the resignation of Yemen's longtime leader, Ali Abdullah Saleh. A U.S.-aided counteroffensive by the government has since pushed the militants back.
Yemen's current president, Abdo Rabby Mansour Hadi, met with U.S. President Barack Obama at the White House on Thursday, where both leaders cited strong counterterrorism cooperation. Earlier this week, Yemen's military reported a U.S. drone strike killed six alleged al-Qaida militants in the group's southern strongholds.
As recently as June, the group's commander, Qasim al-Rimi, released an Arabic-language video urging attacks on U.S. targets and praising the ethnic Chechen brothers accused of carrying out the Boston Marathon bombings. "Making these bombs has become in everyone's ... reach," he said, according to the English subtitles.
"The blinking red intelligence appears to be pointing toward an Al-Qaida in the Arabian Peninsula plot," said Seth Jones, counterterror expert at the Rand Corp., referring to the branch of al-Qaida known as AQAP.
Britain also took action Friday in Yemen, announcing it would close its embassy there on Sunday and Monday as a precaution.
Britain, which closely coordinates on intelligence matters with Washington, stopped short of releasing a similar region-wide alert but added that some embassy staff in Yemen had been withdrawn "due to security concerns." British embassies and consulates elsewhere in the Middle East were to remain open.
Rep. Ed Royce, R-Calif., the House Foreign Affairs Committee chairman, said the embassy threat was linked to al-Qaida and concerned the Middle East and Central Asia.
"In this instance, we can take a step to better protect our personnel and, out of an abundance of caution, we should," Royce said. He declined to say if the National Security Agency's much-debated surveillance program helped reveal the threat.
Rep. C.A. Dutch Ruppersberger of Maryland, the top Democrat on the House Intelligence panel, also supported the department's decision to go public with its concerns.
"The most important thing we have to do is protect American lives," he said, describing the threat as "not the regular chitchat" picked up from would-be militants on the Internet or elsewhere.
The State Department issued another warning a year ago about potential violence connected to the Sept. 11 anniversary. Dozens of American installations were besieged by protests over reports of an anti-Islam video made by an American resident, and in Benghazi, Libya, the U.S. ambassador and three other Americans were killed when militants assaulted a diplomatic post.
The administration no longer says Benghazi was related to the demonstrations. But the attack continues to be a flashpoint of contention with Republicans in Congress who say Obama, former Secretary of State Hillary Rodham Clinton and others in the government misled the country about the nature of the attack after failing to provide adequate diplomatic protection.