By Tony Capra, Andrea Mitchell and Catherine Chomiak, NBC News

A threat of an al Qaeda attack is "real and serious" and "we must not let our guard down," the chairman of the House Permanent Select Committee on Intelligence has warned after a worldwide alert was issued for all U.S. citizens traveling abroad.

The State Department said in a statement issued Friday that warned the terror group and its affiliates "may focus efforts to conduct attacks in the period between now and the end of August."

U.S. officials said that the threat warning is based primarily on a "significant increase in chatter from a growing number of intercepts" throughout the region.

Mike Rogers, the intelligence committee chairman, said the situation showed just how dangerous the al Qaeda network still is.

"The seriousness of the threat stream is a sober reminder of al Qaeda's determination and ongoing intention to commit acts of violence on Western and U.S. targets," he said in a statement late Friday.

"The threats against American interests are real and serious and we must not let our guard down," he added.

At least 22 embassies and consulates that normally open on Sundays -- mostly in the Muslim world -- were closing this weekend because of the potential threat.

The closures so far include diplomatic missions in Cairo, Tel Aviv, Riyadh, Baghdad, Kabul, and Bahrain.

"It is possible we may have additional days of closings as well, depending on our analysis," a State Department official said.

The chairman of the House of Representatives Foreign Affairs Committee, Ed Royce, told MSNBC that he believed it was "probably now prudent, given the fact that, in this case, we do have this intelligence, to take this step to make certain that we have fully protected our embassy personnel."

Britain said it would close its embassy in Yemen on Sunday and Monday. "We are particularly concerned about the security situation in the final days of Ramadan and into Eid," Britain's Foreign Office said in a statement, referring to the Muslim holy month which ends on Wednesday.

The intelligence says nothing about a specific target of the plot, U.S. officials said Friday. The United States is working with foreign spy agencies to try to find out more about the target.

The State Department's alert said 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 of State alerts U.S. citizens to the continued potential for terrorist attacks, particularly in the Middle East and North Africa, and possibly occurring in or emanating from the Arabian Peninsula," it added.

Terrorism expert Xenia Dormandy, U.S. director of London-based think tank Chatham House, said Friday it was the first time in memory that the U.S. had closed such a large number of embassies at once.

"The bottom line is clearly they've had firm intelligence from multiple sources that there's an al Qaeda threat or an al Qaeda-affiliated threat," she said.

Dormandy said the United States' actions were likely influenced by the attack on the U.S. Consulate in Benghazi, Libya, in September in which Ambassador Christopher Stevens and three other Americans were killed.

"I think they are being cautious because any attack on American diplomats is something they want to avoid at all costs," she added. "I think they are certainly likely to be more nervous now about making a mistake than they were before the Libyan [Benghazi consulate] attack, but they've seen the consequences of what can happen if you don't take precautions, so it's perhaps an appropriate response."

NBC News' Ian Johnston and Henry Austin and Reuters contributed to this report.