A flag flutters outside the U.S. embassy in Tel Aviv August 4, 2013.
The United States extended embassy closures by a week in the Middle East and Africa as a precaution on Sunday after an al Qaeda threat that U.S. lawmakers said was the most serious in years.
The State Department said 19 U.S. embassies and consulates would be closed through Saturday "out of an abundance of caution" and that a number of them would have been closed anyway for most of the week due to the Eid celebration at the end of the Muslim holy month of Ramadan.
The United States initially closed 21 U.S. diplomatic posts for the day on Sunday. Some of those will reopen on Monday, including Kabul, Baghdad and Algiers.
Four new diplomatic posts - in Madagascar, Burundi, Rwanda and Mauritius - were added to the closure list for the week.
Last week, the State Department issued a worldwide travel alert warning Americans that al Qaeda may be planning attacks in August, particularly in the Middle East and North Africa.
"There is an awful lot of chatter out there," U.S. Senator Saxby Chambliss, the top Republican on the Senate Intelligence Committee, said on NBC's "Meet the Press."
He said the "chatter" - communications among terrorism suspects about the planning of a possible attack - was "very reminiscent of what we saw pre-9/11."
A National Security Agency surveillance program that electronically collects communications on cellphones and emails - known as intercepts - had helped gather intelligence about this threat, Chambliss said.
It was one of the NSA surveillance programs revealed by former spy agency contractor Edward Snowden to media outlets.
Those programs "allow us to have the ability to gather this chatter," Chambliss said. "If we did not have these programs then we simply wouldn't be able to listen in on the bad guys."
"This is the most serious threat that I've seen in the last several years," Chambliss said.
U.S. military forces in the Middle East region have been on a higher state of alert for the past several days because of the threat, a U.S. official told Reuters on condition of anonymity.
The threat also has prompted some European countries to close their embassies in Yemen, home to an al Qaeda affiliate that is considered one of the most dangerous: al Qaeda in the Arabian Peninsula.
Yemeni soldiers blocked roads around the U.S. and British embassies in Sanaa, while troops with automatic rifles stood outside the French Embassy.
Interpol, the France-based international police agency, on Saturday issued a global security alert advising member states to increase vigilance against attacks after a series of prison breaks inIraq, Libya and Pakistan.
"Al Qaeda is in many ways stronger than it was before 9/11, because it's mutated and it spread and it can come at us from different directions," U.S. Representative Peter King, a Republican, said on ABC's "This Week."
"And al Qaeda in the Arabian Peninsula is probably the most deadly of all the al Qaeda affiliates," he said.
Republicans and Democrats alike on Sunday television talk shows said the threat was serious and sought to defuse the controversy over the NSA surveillance programs, which critics say are an invasion of privacy and civil rights.
"The good news is that we picked up intelligence. And that's what we do. That's what NSA does," U.S. Representative Dutch Ruppersberger, the senior Democrat on the House Intelligence Committee, said on ABC's "This Week."
"We've received information that high-level people from al Qaeda in the Arabian Peninsula are talking about a major attack," he said.
The threat information came just before the Eid celebration at the end of the Muslim holy month of Ramadan later this week and just over a month before the anniversary of al Qaeda's September 11, 2001, attacks on the United States.
A September 11 attack last year killed the U.S. ambassador to Libya and three other Americans in Benghazi.
Republican Senator Lindsey Graham said on CNN's "State of the Union" that the actions taken to close the embassies and issue the global travel alert showed the Obama administration had learned lessons from Benghazi.
"Benghazi was a complete failure. The threats were real there. The reporting was real. And we basically dropped the ball. We've learned from Benghazi, thank God, and the administration is doing this right," he said.