The alert asks American travelers to take extra precautions when traveling overseas and suggests they sign up for State Department alerts and register with consulates or embassies in the countries they are visiting.
The alert was posted a day after the U.S. announced it would close diplomatic facilities on Sunday because of an unspecified threat. 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.
Rep. Ed Royce, R-Calif., chairman of the House Foreign Affairs Committee, said Friday the embassy threat was linked to al-Qaida and focused on the Middle East and Central Asia.
"We've had a series of threats," Royce told reporters. "In this instance, we can take a step to better protect our personnel and, out of an abundance of caution, we should."
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 chit chat" picked up from would-be militants on the Internet or elsewhere.
The State Department issued a major warning last year informing American diplomatic facilities across the Muslim world about potential violence connected to the anniversary of the Sept. 11 terrorist attacks.
Dozens of American installations were besieged by protests over an anti-Islam video made by an American resident.
In Benghazi, Libya, the U.S. ambassador and three other Americans were killed when militants assaulted a diplomatic post, but the administration no longer says that attack was related to the demonstrations.
Friday's alert warned that al-Qaeda or its allies may target U.S. government or private American interests. It cited dangers involved with public transportation systems and other prime sites for tourists, noting that previous terrorist attacks have centered on subway and rail networks as well as airplanes and boats.
"U.S. citizens should take every precaution to be aware of their surroundings and to adopt appropriate safety measures to protect themselves when traveling," the department said. It recommended that American traveling overseas register with consular authorities on a travel registration website. The alert expires on Aug. 31.
Ruppersberger said the U.S. has invested greatly in research and protection system for U.S. embassies and citizens since the Benghazi attack.
"Intelligence is probably the best defense against terrorist attacks," he said. "Intelligence isn't always an exact science, but you have to react and you have to prepare. So we put people on notice and we also put the other side on notice."
Rep. Adam Schiff of California, another Democrat on House intelligence, said he was briefed on the matter this week and called the State Department's decision to close embassies a "prudent" step.
"I hope it will be of short duration," he said.
—By The Associated Press.