WASHINGTON -- The communications chatter among terrorists that prompted the closing of 22 U.S. embassies and consulates Sunday was "reminiscent" of the level prior to the September 11, 2001, terrorist attacks on the United States, Sen. Saxby Chambliss said.
It was "the most serious threat that I've seen in the last several years," Chambliss, a Republican who is vice chairman of the Senate intelligence committee, said in a television interview with NBC News Sunday.
His comments came as the United States, Britain, Germany and France shuttered their embassies in the Yemen capital, Sana'a. Canada made a last-minute decision to close its post in Bangladesh.
The U.S. broadened its closures to 21 other facilities across the Muslim world, from Algiers to Bangladesh.
Some embassies will remain closed on Monday and the U.S. on Sunday released a list of locations that will remain closed through to next Saturday.
In Yemen, soldiers were on high alert searching vehicles in the streets leading to the U.S. and British embassies, epa photographers reported.
Ahead of the U.S. worldwide travel alert issued Friday, communication had been intercepted between senior al-Qaida members discussing plans for operations in the region, according to The New York Times. Yemen, where al-Qaida in the Arabian Peninsula is active, appeared to be a major worry in the current security alert.
Numerous U.S. military units in the Middle East were on a heightened level of alert, CNN reported Sunday, citing a U.S. official. Two other government officials told CNN the U.S. Navy had last week ordered amphibious ships in the Red Sea to move closer to Yemen.
"Chatter means conversation among terrorists about the planning that's going on -- very reminiscent of what we saw pre-9/11," Chambliss said. "We didn't take heed on 9/11 the way that we should, but here, I think it's very important that we do take the right kind of planning."
Experts say the escalating terrorist danger accompanies the end of the Muslim holy month of Ramadan, which comes to a close on or about Wednesday.
The U.S. travel alert through August includes the run-up to the Sept. 11 anniversary of al-Qaida's 2001 airplane attacks against New York and Washington and the first anniversary of the terrorist attack on the U.S. consulate in Benghazi, Libya, that killed the U.S. ambassador and three other diplomats.
Interpol on Saturday noted other August terrorist anniversaries, including the 1998 bombings at the U.S. embassies in Nairobi and Dar es Salaam; the 2008 attacks in Mumbai, India; and the 2003 attack on the JW Marriott Hotel in Jakarta.
Interpol called for increased vigilance after a series of assisted prison breakouts in Iraq, Libya and Pakistan in which al-Qaida involvement was suspected. The breakouts led to the escape of hundreds of terrorists and other criminals."
Security officials at Cairo airport said authorities were on alert for the possible arrival of the escaped militants.
Al-Qaida leader Ayman al-Zawahiri had called earlier for attacks on U.S. interests. In a separate message released Saturday, al-Zawahiri accused the US of "plotting" with the Egyptian army to oust Islamist president Mohammed Morsi last month.
The capture of electronic communication is a major task of the National Security Agency, currently under fire in world capitals and also within the U.S. Congress for revelations by Edward Snowden about the pervasive global data collection.
But the latest round of terrorist warnings bolstered those who defend the practice. Sen. Lindsey Graham, an opposition Republican, told CNN Sunday that gutting the data collection would "make us much less safe, and you're putting our nation at risk."
President Barack Obama, who turned 52 on Sunday, has spent the weekend being kept abreast of the security situation. On Saturday, U.S. National Security Adviser Susan Rice huddled about the situation with Secretary of State John Kerry, Defense Secretary Chuck Hagel, Secretary of Homeland Security Janet Napolitano, the heads of the FBI and CIA and other military and intelligence advisers.