The list includes 15 that were closed 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 Sunday 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 lawmakers 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 state department statement.