The United States said that 19 of its embassies and consulates in the Mideast and Africa would be closed through to August 10 over terror fears.
The list includes 15 that were closed on Sunday, as well as four additional posts. Certain other missions were to reopen Monday, the State Department said.
"This is not an indication of a new threat stream, 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,'' said State Department spokeswoman Jen Psaki in a statement.
At least 25 US embassies and consular offices had initially been ordered closed in response to a terror threat, a move lawmakers said was prompted by intercepts of high-ranking Al-Qaeda operatives signaling a major attack.
In Washington, briefed members of Congress called the intelligence reporting among the most serious they've seen in recent years.
Representative Dutch Ruppersberger, the ranking Democrat on the House Intelligence Committee, told ABC's This Week that Al-Qaeda's "operatives are in place.''
He said the United States knows this "because we've received information that high level people from Al-Qaeda in the Arabian Peninsula are talking about a major attack.''
The chairman of the House Homeland Security Committee, Michael McCaul, called the threat "probably one of the most specific and credible'' he had seen since 9/11.
An attack appeared to be "imminent,'' possibly timed to coincide with the last night of Ramadan, the Muslim holy month, he added.
The diplomatic posts to be closed through Saturday included those in: Abu Dhabi, Amman, Cairo, Riyadh, Dhahran, Jeddah, Doha, Dubai, Kuwait, Manama, Muscat, Sanaa, Tripoli, Antanarivo, Bujumbura, Djibouti, Khartoum, Kigali, and Port Louis.
The new closures are located in Madagascar, Burundi, Rwanda and Mauritius. The outposts that are reopening are located in Iraq, Afghanistan, Bangladesh, Mauritania and Algeria.
The status of US diplomatic missions in Israel - closed Sunday amid the security fears - was not immediately clear, with no mention of them in the State Department statement.
Security was especially tight in Yemen's capital Sanaa on Sunday where Britain, France and Germany also shuttered their embassies in the wake of the US warning.
Special forces with armored personnel carriers were stationed outside the buildings as police and army checkpoints went up on all the city's main thoroughfares.
Residents said they heard the sound of a drone overhead, which could only be American as Washington is the sole power to operate the unmanned aircraft in the region.
Washington considers Al-Qaeda in the Arabian Peninsula to be the jihadist network's most active and dangerous branch, and has waged an intensifying drone war against AQAP militants in Yemen.
In Jordan, authorities beefed up security around the closed US mission.
"Authorities have conducted a sweep for explosives at all US diplomatic locations and beefed up security measures around the US embassy,'' a Jordanian security official told AFP.
Although Washington has responded to terrorist threats before by closing diplomatic missions, this was believed to be the most widespread closure ever.
"I've spent 21 years in the CIA, and I don't think I've ever seen 22 embassies closed simultaneously. This is very, very unusual,'' Robert Baer, a former US case officer in the Middle East, told CNN.
Baer said the US action comes amid an Al-Qaeda resurgence, including prison breaks in Libya and Iraq in which hundreds of inmates have escaped, and turmoil in Egypt, Mali and elsewhere in the region.
Adding further tension, Interpol on Saturday issued a security alert after hundreds of militants were freed in jailbreaks linked to Al-Qaeda.
The worldwide police agency said it suspected Al-Qaeda was involved in jailbreaks across nine countries, notably Iraq, Libya and Pakistan, that had "led to the escape of hundreds of terrorists and other criminals'' in the past month alone.
Meanwhile General Martin Dempsey, chairman of the Joint Chiefs of Staff, told ABC News the threats were "more specific'' than previous threats.
While an exact target was unknown, "the intent seems clear. The intent is to attack Western, not just US, interests,'' Dempsey said.
The State Department late last week issued a worldwide travel alert to US citizens, warning of the "potential for terrorists to attack public transportation systems and other tourist infrastructure.''
Hours after the US alert was issued, an audio recording was posted on militant Islamist forums in which Al-Qaeda chief Ayman al-Zawahiri accused the United States of "plotting'' with Egypt's military, secularists and Christians to overthrow Islamist president Mohamed Morsi.
Washington has been especially cautious about security abroad since an attack on its consulate in Libya's second city Benghazi on September 11 last year.
Ambassador Chris Stevens and three other Americans were killed in the attack blamed on Islamist militants.