The State Department in Washington said the extended closures were "out of an abundance of caution", and not a reaction to a new threat.
Britain said its embassy in Yemen would stay closed until the end of the Muslim festival of Eid on Thursday.
The UK Foreign Office had earlier announced it would shut its mission in the Yemeni capital Sanaa until Tuesday.
Meanwhile US diplomatic missions in Algiers, Kabul and Baghdad are among those which will reopen on Monday, Washington said.
But its diplomatic posts in Abu Dhabi, Amman, Cairo, Riyadh, Dhahran, Jeddah, Doha, Dubai, Kuwait, Manama, Muscat, Sanaa and Tripoli will remain closed until Saturday.
The US state department also added African missions in Antananarivo, Bujumbura, Djibouti, Khartoum, Kigali and Port Louis to the list, meaning a total of 19 US embassies will remain closed this week.
A state department global travel alert, issued on Friday, is in force until the end of August.