The United States reopened some of its diplomatic missions in the Middle East and Africa on Monday, while 19 embassies and consulates in the region remain closed due to security concerns.
Embassies in Baghdad, Kabul, Algiers and Dhaka are among the posts the State Department authorized to open Monday.
But Britain, France, Germany and Norway joined the U.S. in keeping their embassies in Sanaa, the capital of Yemen, closed until later in the week. Yemen is home to al-Qaida in the Arabian Peninsula, a group the U.S. believes is determined to attack Western interests.
In Washington, White House spokesman Jay Carney declined to specify the nature of the threat that led to the embassy closures. But he said, "We believe this threat is serious."
Several key U.S. lawmakers said the threats about a possible imminent attack are the most specific they had seen since al-Qaida's 2001 terrorist attack on the United States.
New York Congressman Peter King said al-Qaida may be stronger now because it has split its operations.
"This is a wakeup call. Al-Qaida is in many ways stronger than it was before 9/11 because it's mutated and it's spread and it came out of some different directions. And al-Qaida in the Arabian Peninsula is probably the most deadly of all the al-Qaida affiliates."
American officials closed the missions and issued a global travel alert for American citizens late last week, warning of a strong potential for terrorist attacks.
The State Department said Sunday it is keeping the 19 remaining posts closed through Saturday "out of an abundance of caution" and not a new threat.
Those missions include sites in Amman, Cairo and Tripoli.
The other posts closed all week are those in Abu Dhabi, Riyadh, Dhahran, Jeddah, Doha, Dubai, Kuwait, Manama, Muscat, Antanarivo, Bujumbura, Djibouti, Khartoum, Kigali and Port Louis.
Other sites opening Monday included the U.S. embassy in Nouakchott and consulates in Herat, Mazar el Sharif, Basrah and Irbil.
U.S. lawmakers and former high-ranking officials called the decision to close diplomatic missions and to issue the global travel alert an extraordinary move.
The international police organization Interpol issued its own security alert.
The U.S. security moves come nearly a year after Islamic militants attacked the U.S. consulate in Benghazi, Libya, killing four Americans, including the U.S. ambassador.