The State Department said Sunday a small number of additional posts will be closed, while others will reopen Monday.
It said the extension was not related to new security threats, but that officials were simply exercising caution.
Sunday's move to close U.S. embassies and consulates came several days after American officials warned of a possible al-Qaida attack.
Most of the more than 20 diplomatic missions are in Muslim countries, where Sunday is a regular business day, and included embassies in Iraq, Libya and Yemen. The embassy in Baghdad will reopen on Monday.
The United States has issued a global travel alert to American citizens that said the potential for terrorist attacks is strong in the Middle East and North Africa.
The State Department says diplomatic posts in the following places will be closed all week: Abu Dhabi, Amman, Cairo, Riyadh, Dhahran, Jeddah, Doha, Dubai, Kuwait, Manama, Muscat, Sanaa, Tripoli, Antanarivo, Bujumbura, Djibouti, Khartoum, Kigali, and Port Louis.
The following embassies and consulates were closed Sunday, but will be allowed to reopen on Monday: Dhaka, Algiers, Nouakchott, Kabul, Herat, Mazar el Sharif, Baghdad, 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.
Other countries took similar actions, and 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.