The US says it will keep a number of embassies in North Africa and the Middle East closed until Saturday, due to a possible militant threat.
Twenty-one US embassies and consulates closed on Sunday.
The state department in Washington said the extended closures were "out of an abundance of caution", and not a reaction to a new threat.
The UK said its embassy in Yemen would stay closed until the Muslim festival of Eid on Thursday.
The decision to close the embassies comes as the US government battles to defend recently disclosed surveillance programmes that have stirred deep privacy concerns.
Security at US diplomatic facilities also remains a concern following last year's attack on the US consulate in Benghazi, Libya, where the US ambassador and three other Americans were killed.
While details of the threats are unspecified, the BBC's David Willis, in Washington, says members of Congress who have been briefed about the intelligence seem to agree it amounts to one of the most serious in recent years - all pointing to the possibility of a major attack, possibly to coincide with the end this week of the Islamic holy month of Ramadan.
A state department global travel alert, issued on Friday, is in force until the end of August.
The department said the potential for an al-Qaeda-inspired attack was particularly strong in the Middle East and North Africa.
Al-Qaeda's affiliate in Yemen, al-Qaeda in the Arabian Peninsula (AQAP), has tried to carry out several high-profile attacks in recent years, including one on Christmas Day in 2009 when a man attempted to blow up a trans-Atlantic jet over Detroit, using explosives sewn into his underwear.
Months earlier, the group tried to kill the Saudi intelligence chief with a bomb on the attacker's body.
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.
Embassies closed on Sunday, a working day in the Muslim world, included Amman, Cairo, Riyadh and Dhaka.
US citizens are advised that all consular appointments have been cancelled and will be rescheduled.
The US embassy in Tel Aviv, Israel, which is normally closed to the public on Sunday, said all its facilities would be shut on Sunday and asked "workers not essential for the building's security" not to come in.
The two consulates in Jerusalem and Haifa were also closed on Sunday.
The embassy closures and US global travel alert came after the US reportedly intercepted al-Qaeda messages.
It has been suggested that they were between senior figures talking about a plot against an embassy.
US lawmakers appearing on Sunday morning shows talked about the threat, saying it was the biggest chatter since 9/11.
"This is the most serious threat that I've seen in the last several years," Republican Senator Saxby Chambliss said on NBC.
"Chatter means conversation among terrorists about the planning that's going on - very reminiscent of what we saw pre-9/11."
Referring to the Middle East, the state department said: "Current information suggests that al-Qaeda and affiliated organisations continue to plan terrorist attacks both in the region and beyond, and that they may focus efforts to conduct attacks in the period between now and the end of August."
The travel alert called for US citizens to be vigilant, warning of "the potential for terrorists to attack public transportation systems and other tourist infrastructure".
Several European countries have also temporarily shut missions in Yemen.
On its website, the UK Foreign Office is advising against all travel to Yemen and is strongly urging British nationals to leave.
It says there is "a high threat from terrorism throughout Yemen" and "a very high threat of kidnap from armed tribes, criminals and terrorists".