The United States has extended the closure of 19 of its embassies and consulates in the Middle East and Africa through August 10, according to the State Department citing "an abundance of caution".
The list includes 15 that were closed on Sunday, as well as four additional posts. Certain other missions were to reopen on Monday, the State Department said.
We've received information that high level people from al-Qaeda in the Arabian Peninsula are talking about a major attack
"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 spokesperson Jen Psaki in a statement.
At least 25 US embassies and consular offices had initially been ordered closed on Sunday in response to a terror threat, a move lawmakers said was prompted by intercepts of high-ranking al-Qaeda operatives signalling a major attack.
In Washington, briefed members of Congress called the intelligence reporting among the most serious they have 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 US 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 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.
|Michael Singh, a former White House official, speaks to Al Jazeera about the heightened security concerns
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 on Sunday amid 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 closed their embassies in the wake of the US warning.
Although Washington has responded to threats before by closing diplomatic missions, this was believed to be the most widespread closure ever.
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".