US diplomatic posts in 19 cities in the Muslim world will be closed at least until the end of this week, the government said, citing "an abundance of caution".
State Department spokeswoman Jen Psaki said the decision to keep the embassies and consulates closed was "not an indication of a new threat", but "merely an indication of our commitment to exercise caution and take appropriate steps to protect our employees, including local employees, and visitors to our facilities".
Diplomatic buildings will remain closed in Egypt, Jordan, Libya, Yemen, Saudi Arabia and Kuwait, among other countries, until Saturday, August 10. The State Department added closures of four African sites, in Madagascar, Burundi, Rwanda and Mauritius.
But the US has also decided to reopen some posts today, including those in Kabul and Baghdad.
The Obama administration announced on Friday that the posts would be closed over the weekend and the State Department announced a global travel alert, warning that al Qaida or its allies might target either US government or private American interests.
The weekend closure of nearly two dozen US diplomatic posts resulted from the gravest terrorist threat seen in years, the top Republican on the Senate Intelligence Committee said.
Senator Saxby Chambliss said "the chatter" intercepted by US intelligence agencies led the government to shut the embassies and consulates and issue a global travel warning to Americans.
"Chatter means conversation among terrorists about the planning that's going on - very reminiscent of what we saw pre-9/11," Mr Chambliss told NBC's Meet The Press.
"This is the most serious threat that I've seen in the last several years."
Dutch Ruppersberger, the top Democrat on the House of Representatives Intelligence Committee, told ABC's This Week that the threat intercepted from "high-level people in al Qaida in the Arabian Peninsula" was about a "major attack".
Yemen is home to al Qaida's most dangerous affiliate, blamed for several notable terrorist plots on the United States. They include the foiled Christmas Day 2009 effort to bomb an airliner over Detroit and the explosives-laden parcels intercepted the following year aboard cargo flights.
Rep Peter King, who leads the House Homeland Security sub-committee on counter-terrorism and intelligence, said the threat included dates but not locations of possible attacks.
"The threat was specific as to how enormous it was going to be and also that certain dates were given," he said on ABC.
Rep Adam Schiff, a House Intelligence Committee member, said the "breadth" of the closures suggested US authorities were concerned about a potential repeat of last year's riots and attacks at multiple embassies, including the deadly assault in Benghazi, Libya, where the US ambassador and three other Americans were killed.
In addition, Interpol, the French-based international policy agency, has issued a global security alert in connection with suspected al Qaida involvement in several recent prison escapes including those in Iraq, Libya and Pakistan.
Those prison breaks added to the concerns about an attack, said Mr Schiff, a Democrat, also noting the approaching end of the Islamic holy month of Ramadan.
"So you have a lot things coming together. ... But all of that would not be enough without having some particularly specific information," he said.
The Obama administration's decision to close the embassies and the politicians' general discussion about the threats come at a sensitive time as the government tries to defend recently disclosed surveillance programmes that have stirred deep privacy concerns and raised the potential of the first serious retrenchment in terrorism-fighting efforts since September 11.
The Senate Judiciary Committee chairman has scoffed at the assertion by the head of the National Security Agency that government methods used to collect telephone and email data have helped foil 54 terror plots.
Mr Schiff said he had seen no evidence linking the latest warnings to that agency's collection of "vast amounts of domestic data".
Others defended the administration's response and promoted the work of the NSA in unearthing the intelligence that lead to the security warnings.
"The bottom line is ... that the NSA's job is to do foreign intelligence," Mr Ruppersburger said. "The whole purpose is to collect information to protect us."
And Mr King, a frequent critic of President Barack Obama, added: "Whether or not there was any controversy over the NSA at all, all these actions would have been taken."
Friday's warning from the State Department urged American travellers to take extra precautions overseas, citing potential dangers involved with transport systems and other prime sites for tourists.
It noted that previous terrorist attacks centred on tube and rail networks as well as planes and boats.