WASHINGTON - The State Department issued a worldwide travel alert for U.S. citizens Friday as it suspended operations in 21 Muslim countries in response to "current information" that suggests al-Qaida-affiliated militant groups might strike within the next month.

Apart from mentioning that an attack might occur in or emanate from the Arabian Peninsula, which is home to one of the most active al-Qaida branches, the State Department's announcement gave few details on the nature of the threat and didn't provide specifics about when or where such an attack might take place.

"They may focus efforts to conduct attacks in the period between now and the end of August," the State Department's travel alert said.

One date in particular - Sunday, Aug. 4 - was mentioned in the State Department warning. All embassies that would have been open were ordered to stay closed that day and perhaps longer. The edict affects embassies and consulates in Muslim countries, where Sunday is a workday, and coincides with a special night in the Islamic holy month of Ramadan.

Rep. Peter King, R-N.Y., who was briefed on the threat, said the focus was on Yemen, the chief staging ground for al-Qaida in the Arabian Peninsula. In a phone interview with CNN, King said the plot was something U.S. authorities had monitored for some time, and that additional information might have come from the visit to Washington this week of Yemen's president, Abed Rabbo Mansour Hadi.

"There's very little doubt, if any, that something serious is being planned," King said, declining to elaborate because the matter involves classified intelligence.

The warning came in the second-busiest international travel month of the year, after July, for Americans. According to U.S. Department of Transportation statistics for 2003 to 2012, 9.1 percent of all international travel takes place in August. American, Southwest, Delta and United airlines said they were monitoring the situation, but none had instituted policies to allow travelers to change their plans without incurring fees.

Analysts who study militant groups said the most popular jihadist online forums hadn't indicated that any major operation was imminent. Commanders typically would plot such attacks face to face or via trusted couriers, never in online chatter, they said.

One discernible trend in the forums, however, is renewed confidence - one analyst called it "jubilation" - after recent prison breaks that freed hundreds of hardened jihadists from prisons in Iraq and Pakistan.

"The one noticeable trend has been increased confident chatter from al-Qaida, prompted by the various prison breaks," said Charles Lister, an analyst at the Terrorism and Insurgency Center of IHS Jane's, a defense research firm in Britain.

Lister and others cautioned that there's no evidence linking the enthusiasm over the prison escapes to the threats that are forcing embassy closures. But the prison breaks certainly have bolstered spirits among al-Qaida-style militants, who suffered heavy losses in years of U.S. and allied targeting and the advent of a pro-democracy movement in the Arab world.

Now jihadists are leading the rebel fight in Syria, gaining operational space in the lawlessness of transitional North African states and regrouping in Iraq, where senior members of al-Qaida were among the hundreds of detainees to escape last month from the notorious Abu Ghraib prison during an insurgent attack.

That assault was the latest of seven prison attacks since al-Qaida's emir in Iraq, Abu Bakr al-Baghdadi, announced his "Breaking the Walls" campaign a year ago to free his comrades, according to the Institute for the Study of War, a research center in Washington. The recent breaks have "significantly increased the operational depth" of al-Qaida in Iraq, the institute said in a report issued Sunday.

Peter Bergen, a leading global terrorism expert and national security analyst for CNN, wrote Thursday that a 2006 prison break in the Yemeni capital of Sanaa effectively created al-Qaida's branch in the Arabian Peninsula, the area singled out Friday in the U.S. government's travel alert. Two of the 23 inmates who crawled to freedom through a 460-foot tunnel went on to become the leader and second in command for al-Qaida in the Arabian Peninsula.

"The attacks are generally well-organized and often free significant numbers of inmates, refreshing the militant groups' ranks, and each successful prison break is a propaganda coup," Bergen wrote.

Analysts say there's too little public information to draw conclusions about the reasons for such a widespread shutdown of diplomatic operations. Such precautionary measures have become increasingly common in the aftermath of the attacks last Sept. 11 that killed Ambassador Chris Stevens and three other Americans at U.S. posts in the eastern Libyan city of Benghazi.

A similar "worldwide caution" to American travelers was issued in February. That warning said, "These attacks may employ a wide variety of tactics including suicide operations, assassinations, kidnappings, hijackings and bombings." That month a suicide bomber blew himself up outside the U.S. Embassy in Ankara, Turkey, killing a security guard.